Refactor: Avoid hardcoding boost::ecuyer1988

[WardBrian]

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

This is the first part of #3256. After this, we can update the signatures in stanc3 to use stan::rng_t, which will allow us to change what is used underlyingly without breaking all our CI.

Intended Effect

Avoid using the name boost::ecuyer1988 directly. We refer to it by an alias stan::rng_t, which allows the underlying type to be more easily changed in the future.

How to Verify

Side Effects

None

Documentation

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):

Simons Foundation

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/)

[WardBrian]

Ok, my proposed order of merges is:

1. Merge this, which has no semantic changes, just some new typenames.
2. Merge Avoid hardcoding boost::ecuyer1988 stanc3#1395 which uses those typenames
3. Open a PR from https://github.com/stan-dev/stan/tree/refactor/actually-switch-rngs which actually switches to mixmax

[SteveBronder]

lgtm!