Complex support for more linear algebra functions

This is based on https://github.com/stan-dev/stanc3/issues/1162

The following functions do not fully compile in expression tests when added to stanc. I've attached gists which show the compiler errors.

• operator\(complex_matrix, complex_vector), operator\(complex_matrix, complex_matrix) (aka mdivide_left)
  https://gist.github.com/WardBrian/6babcd2fd0b4e1bb5b311fa9fc5c8baa
• determinant(complex_matrix)
  https://gist.github.com/WardBrian/5bbe128aa159d9268a6acaea08f23187
• inverse(complex_matrix)
  https://gist.github.com/WardBrian/2552a803a0b7a1575317b4f3b3f55893
• generalized_inverse(complex_matrix) (Bob: don't even know if this one's possible)
  https://gist.github.com/WardBrian/be2edbb34a9fcbccd4bf4927e7a723e1
• cholesky_decompose(complex_matrix)
  https://gist.github.com/WardBrian/af8be400f4de3a4ca6646d0eaf4a862c (this one passes expression tests, fails the included test model)
• svd_U(complex_matrix)
  https://gist.github.com/WardBrian/f22051278084bf499ba77d7c165de422
• svd_V(complex_matrix)
  https://gist.github.com/WardBrian/d9f416cfabaae205b7b0a77a8dd1a8bd
• operator./(CM, complex), operator./(complex, CM), operator./=(T, T) for T in CM (aka elt_divide)
  https://gist.github.com/WardBrian/8911027041550763eda66d6162b2b7a5
• singular_values(complex_matrix)
  https://gist.github.com/WardBrian/fa245b129b93a02d8a3e5f288dc32cd8
• eigenvalues_sym(complex_matrix)
  https://gist.github.com/WardBrian/98965f46b023c5afbdf4ae33fb8d1662
• eigenvectors_sym(complex_matrix)
  https://gist.github.com/WardBrian/0341519bfaa0c215c1f9c00f7d852f0e
• subtract(complex, complex_matrix) => complex_matrix
  https://gist.github.com/WardBrian/0a9cf4651a844165e325fbb208fde7dd
• add(complex, complex_matrix) => complex_matrix
  https://gist.github.com/WardBrian/67f3f279d19c386998caaafac36c0cf9
• abs(CV) => CV

Start by reproducing the compiler errors from the linked gists in the expression tests, then inspect the existing complex linear algebra implementations corresponding to the unchecked functions. Done means the remaining listed functions compile in expression tests and the included cholesky test model passes, with the generalized inverse question resolved.