Add get_param_sizes function to generated C++

[WardBrian]

This PR splits off some small changes from #1100 and adds a new function called get_param_sizes with signature:

std::vector<size_t> get_param_sizes(const bool emit_transformed_parameters__ = true,
                                    const bool emit_generated_quantities__ = true)

This function is forward-compatible with all future types we have discussed for Stan (they will always have a size), and makes it possible to re-write things like random_var_context to not depend on get_dims, which is not forward compatible and assumes rectangular dimensions.

Submission Checklist

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

Release notes

Added code-generation for a new function get_param_sizes

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)

[WardBrian]

There is a corresponding C++ branch here: https://github.com/stan-dev/stan/tree/model-base-get-sizes/

This isn't strictly necessary for tuples (this final usage of get_dims can be hacked around) so I'm happy to avoid it for the time being.