Reverse slices and inequality slices with implicit parts

[edemaine]

This was mostly for fun, and for code golfers to be able to reverse a string with x[..>=]. It ended up a little bigger than I expected, to get the many cases working...

• Slices remain increasing by default (even without </<= modifier)
• With explicit > or >=, slices are reversed, similar to a range, so normally left ≥ right. For example, x[..>=] is equivalent to x[-1 ..>= 0] which is equivalent to x[x.length - 1 ..>= 0]
• Also fix increasing slices with implicit parts, like x[..<]

[STRd6]

Golfers are going to go nuts over x[..<=]