Clean up module interfaces

[WardBrian]

This does a few things:

1. Closes .mli file usage #358 by deleting the 'index' modules which dune creates for us, like Middle.ml
2. Adds interfaces and documentation for a couple more existing modules, namely Canonicalize and Stan_math_signatures
3. Removes Frontend_utils by finding better homes for what lived there (basically all of it's functions were only called in one place and not generalizable)

Submission Checklist

• Run unit tests
• Documentation
  • OR, no user-facing changes were made

Release notes

Cleaned up the internal structuring of modules in the compiler

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[rok-cesnovar]

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