Use boost beta, simplify overloads

[andrjohns]

Summary

This PR replaces our manual implementation of the beta function with the one from Boost which is more accurate over a broader domain

I've also combined the multiple overloads into a single overload per prim, rev, fwd now that we have if constexpr.

Tests

N/A

Side Effects

N/A

Release notes

Use Boost's implementation of beta for improved stability

Checklist

• [x] Copyright holder: Andrew Johnson

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the 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/)

• [x] the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• [x] the code is written in idiomatic C++ and changes are documented in the doxygen

• [x] the new changes are tested

[andrjohns]

@SteveBronder what do you think of this structure for combining overloads with if constexpr? Just figuring out a good consistent structure to follow