Improve BindableList performance + reduce allocations

[smoogipoo]

Extracted out from + excludes the controversial changes of #6405.

Before:

|    Method | NumBindings |        Mean | Error |     StdDev | Ratio | RatioSD |   Gen0 |   Gen1 | Allocated | Alloc Ratio |
|---------- |------------ |------------:|------:|-----------:|------:|--------:|-------:|-------:|----------:|------------:|
|    Create |           0 |    81.33 ns |    NA |   1.539 ns |  1.00 |    0.00 | 0.0930 |      - |     584 B |        1.00 |
|       Add |           0 |   135.18 ns |    NA |  21.728 ns |  1.66 |    0.24 | 0.1364 |      - |     856 B |        1.47 |
|    Remove |           0 |    96.40 ns |    NA |   1.789 ns |  1.19 |    0.04 | 0.1147 |      - |     720 B |        1.23 |
|     Clear |           0 |   106.17 ns |    NA |   0.695 ns |  1.31 |    0.02 | 0.1211 |      - |     760 B |        1.30 |
|  AddRange |           0 |   131.20 ns |    NA |   2.461 ns |  1.61 |    0.06 | 0.1390 |      - |     872 B |        1.49 |
|  SetIndex |           0 |   126.74 ns |    NA |   1.458 ns |  1.56 |    0.05 | 0.1440 |      - |     904 B |        1.55 |
| Enumerate |           0 |    83.12 ns |    NA |   0.102 ns |  1.02 |    0.02 | 0.0930 |      - |     584 B |        1.00 |
|           |             |             |       |            |       |         |        |        |           |             |
|    Create |           1 |   232.07 ns |    NA |  11.904 ns |  1.00 |    0.00 | 0.1299 |      - |     816 B |        1.00 |
|       Add |           1 |   337.81 ns |    NA |   9.747 ns |  1.46 |    0.03 | 0.1884 |      - |    1184 B |        1.45 |
|    Remove |           1 |   238.13 ns |    NA |   1.806 ns |  1.03 |    0.06 | 0.1450 |      - |     912 B |        1.12 |
|     Clear |           1 |   256.45 ns |    NA |   0.311 ns |  1.11 |    0.06 | 0.1578 |      - |     992 B |        1.22 |
|  AddRange |           1 |   358.29 ns |    NA |   0.575 ns |  1.55 |    0.08 | 0.1936 |      - |    1216 B |        1.49 |
|  SetIndex |           1 |   344.04 ns |    NA |   1.081 ns |  1.48 |    0.08 | 0.2036 |      - |    1280 B |        1.57 |
| Enumerate |           1 |   230.97 ns |    NA |   3.169 ns |  1.00 |    0.06 | 0.1299 |      - |     816 B |        1.00 |
|           |             |             |       |            |       |         |        |        |           |             |
|    Create |          10 | 1,454.40 ns |    NA |  18.072 ns |  1.00 |    0.00 | 0.6485 |      - |    4072 B |        1.00 |
|       Add |          10 | 2,080.50 ns |    NA |   1.270 ns |  1.43 |    0.02 | 0.9384 |      - |    5888 B |        1.45 |
|    Remove |          10 | 1,369.26 ns |    NA |   4.166 ns |  0.94 |    0.01 | 0.6065 |      - |    3808 B |        0.94 |
|     Clear |          10 | 1,447.20 ns |    NA |   2.785 ns |  1.00 |    0.01 | 0.6771 |      - |    4248 B |        1.04 |
|  AddRange |          10 | 2,223.23 ns |    NA |   6.040 ns |  1.53 |    0.01 | 0.9651 |      - |    6064 B |        1.49 |
|  SetIndex |          10 | 2,176.85 ns |    NA |  20.808 ns |  1.50 |    0.03 | 1.0223 |      - |    6416 B |        1.58 |
| Enumerate |          10 | 1,423.40 ns |    NA |   0.864 ns |  0.98 |    0.01 | 0.6485 |      - |    4072 B |        1.00 |
|           |             |             |       |            |       |         |        |        |           |             |
|    Create |          20 | 2,960.90 ns |    NA |  75.523 ns |  1.00 |    0.00 | 1.2703 | 0.0038 |    7976 B |        1.00 |
|       Add |          20 | 4,214.74 ns |    NA |  11.631 ns |  1.42 |    0.04 | 1.8387 |      - |   11544 B |        1.45 |
|    Remove |          20 | 3,661.51 ns |    NA |   4.385 ns |  1.24 |    0.03 | 1.1635 |      - |    7312 B |        0.92 |
|     Clear |          20 | 3,072.85 ns |    NA | 158.676 ns |  1.04 |    0.08 | 1.2970 | 0.0038 |    8152 B |        1.02 |
|  AddRange |          20 | 4,493.25 ns |    NA |   3.102 ns |  1.52 |    0.04 | 1.8921 | 0.0076 |   11880 B |        1.49 |
|  SetIndex |          20 | 4,488.73 ns |    NA |  48.956 ns |  1.52 |    0.02 | 1.9989 |      - |   12552 B |        1.57 |
| Enumerate |          20 | 3,041.22 ns |    NA | 160.420 ns |  1.03 |    0.03 | 1.2703 | 0.0038 |    7976 B |        1.00 |

After:

|    Method | NumBindings |        Mean | Error |     StdDev | Ratio | RatioSD |   Gen0 | Allocated | Alloc Ratio |
|---------- |------------ |------------:|------:|-----------:|------:|--------:|-------:|----------:|------------:|
|    Create |           0 |    49.91 ns |    NA |   0.813 ns |  1.00 |    0.00 | 0.0561 |     352 B |        1.00 |
|       Add |           0 |    74.59 ns |    NA |   1.424 ns |  1.49 |    0.00 | 0.0842 |     528 B |        1.50 |
|    Remove |           0 |    75.69 ns |    NA |   0.761 ns |  1.52 |    0.04 | 0.0842 |     528 B |        1.50 |
|     Clear |           0 |    74.06 ns |    NA |   0.168 ns |  1.48 |    0.02 | 0.0842 |     528 B |        1.50 |
|  AddRange |           0 |    82.64 ns |    NA |   1.324 ns |  1.66 |    0.05 | 0.0842 |     528 B |        1.50 |
|  SetIndex |           0 |    74.48 ns |    NA |   0.211 ns |  1.49 |    0.02 | 0.0842 |     528 B |        1.50 |
| Enumerate |           0 |    50.59 ns |    NA |   0.092 ns |  1.01 |    0.02 | 0.0561 |     352 B |        1.00 |
|           |             |             |       |            |       |         |        |           |             |
|    Create |           1 |   161.05 ns |    NA |   1.265 ns |  1.00 |    0.00 | 0.0560 |     352 B |        1.00 |
|       Add |           1 |   241.94 ns |    NA |   1.801 ns |  1.50 |    0.00 | 0.0839 |     528 B |        1.50 |
|    Remove |           1 |   193.39 ns |    NA |   1.447 ns |  1.20 |    0.02 | 0.0842 |     528 B |        1.50 |
|     Clear |           1 |   190.15 ns |    NA |   1.779 ns |  1.18 |    0.00 | 0.0842 |     528 B |        1.50 |
|  AddRange |           1 |   262.38 ns |    NA |   0.329 ns |  1.63 |    0.01 | 0.0839 |     528 B |        1.50 |
|  SetIndex |           1 |   244.24 ns |    NA |   0.791 ns |  1.52 |    0.01 | 0.0839 |     528 B |        1.50 |
| Enumerate |           1 |   163.61 ns |    NA |   5.447 ns |  1.02 |    0.03 | 0.0560 |     352 B |        1.00 |
|           |             |             |       |            |       |         |        |           |             |
|    Create |          10 | 1,623.94 ns |    NA |  80.347 ns |  1.00 |    0.00 | 0.2422 |    1520 B |        1.00 |
|       Add |          10 | 1,588.68 ns |    NA |  36.029 ns |  0.98 |    0.07 | 0.3624 |    2280 B |        1.50 |
|    Remove |          10 | 1,713.21 ns |    NA | 524.839 ns |  1.05 |    0.27 | 0.2689 |    1696 B |        1.12 |
|     Clear |          10 | 1,309.85 ns |    NA |  90.409 ns |  0.81 |    0.10 | 0.2689 |    1696 B |        1.12 |
|  AddRange |          10 | 1,710.17 ns |    NA |   4.467 ns |  1.05 |    0.05 | 0.3624 |    2280 B |        1.50 |
|  SetIndex |          10 | 1,629.28 ns |    NA |  17.520 ns |  1.00 |    0.04 | 0.3624 |    2280 B |        1.50 |
| Enumerate |          10 | 1,269.36 ns |    NA |  58.352 ns |  0.78 |    0.07 | 0.2422 |    1520 B |        1.00 |
|           |             |             |       |            |       |         |        |           |             |
|    Create |          20 | 2,184.72 ns |    NA |   4.066 ns |  1.00 |    0.00 | 0.4921 |    3104 B |        1.00 |
|       Add |          20 | 3,328.71 ns |    NA |  15.992 ns |  1.52 |    0.00 | 0.7401 |    4656 B |        1.50 |
|    Remove |          20 | 2,346.84 ns |    NA |   9.699 ns |  1.07 |    0.01 | 0.5226 |    3280 B |        1.06 |
|     Clear |          20 | 2,206.18 ns |    NA |   5.316 ns |  1.01 |    0.00 | 0.5226 |    3280 B |        1.06 |
|  AddRange |          20 | 4,812.98 ns |    NA |  75.557 ns |  2.20 |    0.04 | 0.7401 |    4656 B |        1.50 |
|  SetIndex |          20 | 3,316.43 ns |    NA |  60.392 ns |  1.52 |    0.02 | 0.7401 |    4656 B |        1.50 |
| Enumerate |          20 | 2,168.55 ns |    NA |   4.329 ns |  0.99 |    0.00 | 0.4921 |    3104 B |        1.00 |