Add folding, subtraction, and division combinators

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[codecov-commenter]

Codecov Report

Merging #7 (df73e0f) into main (f171cdf) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
- Coverage   99.17%   99.16%   -0.01%     
==========================================
  Files          17       16       -1     
  Lines         364      361       -3     
==========================================
- Hits          361      358       -3     
  Misses          3        3              

Impacted Files	Coverage Δ
src/redex/combinator/__init__.py	100.00% <100.00%> (ø)
src/redex/combinator/_fold.py	100.00% <100.00%> (ø)
src/redex/combinator/_residual.py	100.00% <100.00%> (ø)

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