Refactor cmm functions to be more generic over operand width

[jvanburen]

No description provided.

[github-actions]

Selection Change Check

This PR modifies the original selection pass (targeting Mach).
Please check whether the changes should also be applied to the
CFG variant of the pass.

[jvanburen]

This PR refactors cmm_helpers in preparation for unboxed 8 and 16-bit integers to be added to the compiler.
It replaces many functions that were copied for each width with a single more generic version, and then updates to_cmm to use it.

[github-actions]

Parser Change Checklist

This PR modifies the parser. Please check that the following tests are updated:

• parsetree/source_jane_street.ml

This test should have examples of every new bit of syntax you are adding. Feel free to just check the box if your PR does not actually change the syntax (because it is refactoring the parser, say).

[Gbury]

Seems good overall, modulo the few changes which change/allow more things in mixed blocks, and which don't seem to be part of the refactor (and which would need more in-depth review to make sure the code is correct and that the rest of the code dealing with them is correct for these new values).

[TheNumbat]

lgtm; some comments on comments