codegen: use stan::io::read_from_context

[WardBrian]

Alternative to #1734 which uses stan-dev/stan#3438

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

Fixed several issues compiling data blocks with arrays containing tuples which would lead to mangled numbers or crashes.

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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.31%. Comparing base (1f016e3) to head (2f09bce).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1742      +/-   ##
==========================================
- Coverage   92.35%   92.31%   -0.05%     
==========================================
  Files          70       70              
  Lines       10491    10292     -199     
==========================================
- Hits         9689     9501     -188     
+ Misses        802      791      -11     

Files with missing lines	Coverage Δ
src/middle/Stmt.ml	93.98% <ø> (+3.37%)	⬆️
src/stan_math_backend/Lower_expr.ml	96.98% <100.00%> (-0.08%)	⬇️
src/stan_math_backend/Lower_stmt.ml	97.54% <100.00%> (+0.01%)	⬆️
src/stan_math_backend/Transform_Mir.ml	94.82% <100.00%> (-0.83%)	⬇️

... and 2 files with indirect coverage changes

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[SteveBronder]

This feels very satisfying