Proto/Michelson: fix a bug in compare_comparable

Context

This MR fixes a bug in Michelson comparison function and adds QCheck tests to ensure that the function now behaves as it did before the refactoring that introduced the bug (in !2993 (merged)).

Manually testing the MR

To run the new tests:

$ dune build @src/proto_alpha/lib_protocol/runtest_test_script_comparison

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR