Move assignment logic into promotion functionality

[WardBrian]

Follow on to #1004. This tries to incorporate one of @nhuurre's suggestions there to handle array assignment and literals using the Promote expression. I'm thinking Tuples will need something like this as well, if we want to assign (int,int) to (real, int) for example.

I'm not 100% confident this will work as-is (mainly due to my lack of familiarity with the C++ at hand), so I am making this draft to run the tests.

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Refactor how arrays have their types promoted internally.

Copyright and Licensing

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license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[WardBrian]

@rok-cesnovar this requires more work than I thought, ignore the review and we can revisit after 2.29

[WardBrian]

This should be ready for review. A brief summary of changes:

1. Both array expressions and assignment now use the Promotion expression during typechecking.
2. The code formerly in SignatureMismatch for promotion logic has been moved to its own Promotion module
3. In code generation, assignment assumes that C++ will do one level of promotion so it strips away one layer to avoid doing duplicate work (e.g., double x = 1; is valid C++, so double x = promote_scalar<double>(1); is just silly)
4. Complex scalars were changed to use = instead of assign (I think this was an oversight before? If there's a reason for it it was not clear to me, as the changed code still compiles)
5. The special-cased code for handling array expressions in codegen was removed, it is all done with Promotion now.
6. I tried my best to remove promotion expressions from things like the partial evaluator -- if we can evaluate it in OCaml directly, we don't need to keep the Promotion information around.

[WardBrian]

@rok-cesnovar Would you mind taking a look at this now that the release is out? Thanks!