Stanc.Std: Move stan_math_backend

[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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[codecov]

Codecov Report

❌ Patch coverage is 92.98246% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.16%. Comparing base (8c29526) to head (922ca72).

Files with missing lines	Patch %	Lines
src/std/Hash_set.ml	44.44%	5 Missing ⚠️
src/stan_math_backend/Transform_Mir.ml	95.00%	2 Missing ⚠️
src/stan_math_backend/Lower_program.ml	85.71%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           std/frontend    #1660      +/-   ##
================================================
- Coverage         92.21%   92.16%   -0.05%     
================================================
  Files                69       70       +1     
  Lines             10099    10124      +25     
================================================
+ Hits               9313     9331      +18     
- Misses              786      793       +7     

Files with missing lines	Coverage Δ
src/stan_math_backend/Cpp.ml	90.90% <100.00%> (ø)
src/stan_math_backend/Cpp_Json.ml	100.00% <100.00%> (ø)
src/stan_math_backend/Lower_expr.ml	97.75% <100.00%> (+0.28%)	⬆️
src/stan_math_backend/Lower_functions.ml	97.88% <100.00%> (+0.09%)	⬆️
src/stan_math_backend/Lower_stmt.ml	97.52% <100.00%> (ø)
src/stan_math_backend/Mangle.ml	100.00% <100.00%> (ø)
src/stan_math_backend/Numbering.ml	97.95% <100.00%> (ø)
src/std/std.ml	89.69% <100.00%> (+2.19%)	⬆️
src/stan_math_backend/Lower_program.ml	99.20% <85.71%> (+0.01%)	⬆️
src/stan_math_backend/Transform_Mir.ml	95.54% <95.00%> (-0.21%)	⬇️
... and 1 more

... and 1 file with indirect coverage changes

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