Consolidate check_compatible_arguments_mod_conv functions

[WardBrian]

Originally discussed as part of #998:

@nhuurre writes:

The only difference between UnsizedType and SignatureMismatch versions of check_compatible_arguments_mod_conv should be that one returns a boolean (yes/no answer) and the other constructs a type mismatch info object (for printing a detailed error). The depth parameter is not visible outside the module. It is only used to control the behavior when check_compatible_arguments recurses into function types.

It is indeed quite awkward to have two separate functions that do the same thing.

At the moment. UnsizedType.check_compatible_arguments_mod_conv is only used in Stan_math_signatures to check that operators like +, -, *, etc are well-typed. We can very easily adapt this to use the SignatureMismatch version, though it requires some untangling to prevent a circular dependency among the files.

[adamhaber]

I can try this. Can you explain what you mean by

though it requires some untangling to prevent a circular dependency among the files.

?

[WardBrian]

Currently, SignatureMismatch depends on Stan_math_signatures. If we try to use SignatureMismatch inside Stan_math_signatures, we get a loop which is impossible to compile. It just requires rethinking or moving some things around

This should definitely happen after #999, and maybe even after #995

[adamhaber]

Got it. Would be happy to try after they're merged.