Fix requirements inside loops and functions

[dfremont]

Description

This PR makes require statements work as expected inside loops and functions (currently they can trigger an assertion failure or evaluate improperly).

Issue Link

Fixes #313.

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.43%. Comparing base (4ae8ee2) to head (ceda6c5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #316      +/-   ##
==========================================
+ Coverage   91.38%   91.43%   +0.04%     
==========================================
  Files          54       54              
  Lines       13555    13575      +20     
==========================================
+ Hits        12387    12412      +25     
+ Misses       1168     1163       -5     

Files with missing lines	Coverage Δ
src/scenic/core/dynamics/scenarios.py	94.33% <100.00%> (-0.02%)	⬇️
src/scenic/core/regions.py	87.30% <ø> (-0.36%)	⬇️
src/scenic/core/requirements.py	95.91% <100.00%> (+0.68%)	⬆️
src/scenic/syntax/compiler.py	98.47% <ø> (ø)

... and 4 files with indirect coverage changes

[Eric-Vin]

Looks good!