Make contains_std_vector variadic.

[cqfd]

Summary

This is a small change that reimplements the contains_std_vector metafunction with variadic templates. The previous implementation used a helper metafunction, is_std_vector, but I think it's a little simpler to do that logic directly in contains_std_vector. So, I've also removed is_std_vector and its tests (they weren't testing anything very interesting).

These changes are along the lines of things we've been discussing on Discourse: https://discourse.mc-stan.org/t/stan-math-library-c-code-modernization-ideas/5293/8.

I didn't go this far in this PR, but the pattern of checking if anything in a parameter pack satisfies some type-level predicate (in this case, if something is a std::vector) presumably comes up a lot, and could be handled in a more general way, e.g. https://gist.github.com/cqfd/aae7a1d0721279e5e1e04ad8a7205d62

Tests

No new tests (in fact, fewer tests). There are a few tests that used to reference is_std_vector, which I've tweaked to use contains_std_vector (it's variadic, so it handles a single argument like is_std_vector just fine).

Side Effects

None.

Checklist

• Math issue #(issue number)

• Copyright holder: (fill in copyright holder information)

  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)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

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

• [x ] the new changes are tested

[seantalts]

cc @bob-carpenter @bbbales2 - I like this new style! further examples of modernization.

[syclik]

Sweet! This really does make it a lot cleaner. Thanks!