Peephole optimization for commonly paired functions

See #1674 for an initial attempt. Thanks to @sims1253 for the suggestion

There are several functions that historically duplicated work and now can be done in a single call returning a tuple. These were primarily added in https://github.com/stan-dev/math/pull/2931:

• qr: replaces calls to qr_Q/qr_R
• qr_thin: replaces calls to qr_thin_Q/qr_thin_R
• eigendecompose_sym: replaces calls to eigenvalues_sym/eigenvectors_sym
• eigendecompose: replaces calls to eigenvalues/eigenvectors
• complex_schur_decompose: replaces calls to complex_schur_decompose_t/complex_schur_decompose_u
• svd: replaces calls to svd_U/svd_V
• csr_extract : replaces calls to csr_extract_u/csr_extract_v/csr_extract_w

The proposal is to add a small optimization which detects cases where these are called one after the other and rewrite them to use the combined version.

It may also be useful to separately flag a warning for models where the older functions are both called on the same thing.

Start by reading issue #1674 and the stanc3 optimization code that handles function-call patterns. Trace how paired calls to qr, qr_thin, eigendecompose, complex_schur_decompose, svd, or csr_extract are represented and transformed. Done means qualifying consecutive calls use the combined tuple-returning function, with coverage for the rewrite and possibly a separate warning for older paired calls.