Stanc.Std: Move over stan_math_signatures

[WardBrian]

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Replace this text with a short note on what will change if this pull request is merged. This will be included in the release notes.

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)

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[WardBrian]

Unfortunately Jenkins doesn't currently run on each piece of a stack properly, c.f. jenkinsci/github-branch-source-plugin#1545

[codecov]

Codecov Report

❌ Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.27%. Comparing base (48aa147) to head (cd739f7).

Files with missing lines	Patch %	Lines
src/std/std.ml	69.23%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           start-std    #1657      +/-   ##
=============================================
- Coverage      92.31%   92.27%   -0.04%     
=============================================
  Files             68       68              
  Lines           9975     9991      +16     
=============================================
+ Hits            9208     9219      +11     
- Misses           767      772       +5     

Files with missing lines	Coverage Δ
src/stan_math_signatures/Stan_math_signatures.ml	86.66% <100.00%> (+0.39%)	⬆️
src/std/std.ml	73.33% <69.23%> (-26.67%)	⬇️

... and 1 file with indirect coverage changes

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