Completed the hello-world and leap exercises.
Updated configuration.

Completed the difference-of-squares practice exercise, and added the
`bin/format-all.sh` script that recursively searches for .odin files and
runs `odinfmt` on them.

All source files have been reformatted with the `odinfmt.json` spec.
`bin/format-all.sh` is now run automatically on commit.

The `test.yml` GitHub workflow now pushes any changes that it makes, so
that they persist in the repo.

`test.yml` now only attempts a commit and push if the automated
formatting actually changed any files.

The verify-exercises script now runs the individual test for both
concept and practice exercises, and is the default in the GitHub
workflow.

Tests were lifted from the Gleam track.

- The README now contains more thorough instructions on how to add a new
  exercise.
- Fixed a variable name in the test runner that caused the "test all
  solutions" loop to run multiple times when verifying solutions.

- Fixed bin/run-test.sh so that it doesn't delete the stub solution when
  a test fails.
- Restored exercises, and added a NotImplemented error condition for the
  stubs.

- Ran the code formatter on source files.
- Rehaul of the README to follow Python's example.

The workflow now uses a fixed commit hash to pull a specific release of
Odin. We now use Ubuntu 22.04 which has the libffi8 dependency, and add
clang as the only other dependency.

Added a check to bin/run-test.sh so that verifies that the stub solution
does *not* pass any tests. Fixed hello-world.odin to actually be a stub.

- Improved bin/run-test.sh to add more visible color coding to the
  output.
- Replaced NotImplemented error codes with a compile-time panic for
  procedures that have not yet been implemented.
- Fixed stubs that were actually complete examples.

- Exercises' unit tests were modified so that only a minimal subset of
  tests are initially enabled; the rest are skipped.
- The test runner was modified to automatically unskip all test cases
  before running the tests.