Tangled includes

[syclik]

Summary:

Currently, there are rev and fwd headers included in prim. We want to separate the includes.
We also want to separate out the mat, arr, scal includes within those as well.

Reproducible Steps:

> grep -rl "rev" stan/math/prim

shows a list of reverse mode files that are included in prim.

There is also:
> make test-math-dependencies
that provides some information about the tangled includes.

Additional Information:

This issue should also doc the test-math-dependencies.

Current Version:

v2.9.0