Place template guards on prim/core operator overloads

[WardBrian]

Summary

We overload the basic operators (*, /, -, +) in math/prim/core, but we don't have any specific requirements on what the types are. This leads to issues like those noted in #2699 with signatures like subtract(complex, complex_matrix) => complex_matrix, and I believe it is also the source of some of the other template failures.

Tests

Existing tests are maintained, I've also tested that this allows subtract(complex, complex_matrix) => complex_matrix and add(complex, complex_matrix) => complex_matrix, which will be added to the expression tests following this.

Side Effects

I do not believe so. Any place where these overloads were being used and a Stan scalar was not the internal type was most likely incorrect beforehand.

Release notes

Checklist

• Math issue: Related to Complex support for more linear algebra functions #2699

• Copyright holder: Simons Foundation

  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/)

• 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)

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

• the new changes are tested

[WardBrian]

This PR also fixes expression tests for the complex signatures of:

singular_values
svd_V
svd_U
elt_divide
eigenvalues_sym
eigenvectors_sym

It appears all of these were broken due to picking up incorrect overloads for operators from these files.

[rok-cesnovar]

Clean and simple. Thanks!