Reuse compiled model methods across models

[andrjohns]

Submission Checklist

• [x] Run unit tests
• [x] Declare copyright holder and agree to license (see below)

Summary

Updated implementation of #894.

Now that we use the standard Windows toolchain, I can resurrect the PR for caching the compilation of model methods. Users will now only have to compile the model methods once for one model, and future calls for other models to $init_model_methods() will simply link the existing cmdstan model object against the cached compiled model methods.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)