Add tuple-returning special functions

[WardBrian]

Counterpart to https://github.com/stan-dev/math/pull/2931

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] If a user-facing facing change was made, the documentation PR is here: https://github.com/stan-dev/docs/pull/664

Release notes

Exposed new functions qr_thin, eigendecompose_sym, eigendecompose, complex_schur_decompose, svd, and csr_extract.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov[bot]]

Codecov Report

Merging #1346 (d269060) into master (0e32ba1) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1346      +/-   ##
==========================================
+ Coverage   89.27%   89.29%   +0.01%     
==========================================
  Files          65       65              
  Lines       10615    10626      +11     
==========================================
+ Hits         9477     9488      +11     
  Misses       1138     1138              

Files Changed	Coverage Δ
src/middle/Stan_math_signatures.ml	98.06% <100.00%> (+0.01%)	:arrow_up: