Add support for the poly transform.

[matthewwardrop]

Closes #37 .

This PR adds support for the poly transform. It is not directly usable yet in a formula because there are some incoming changes which need to land to enable upcasting of arrays to columns/etc.

[codecov]

Codecov Report

Merging #44 (7caa9cd) into main (279b97c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #44   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           39        41    +2     
  Lines         1447      1517   +70     
=========================================
+ Hits          1447      1517   +70     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
formulaic/materializers/base.py	100.00% <100.00%> (ø)
formulaic/materializers/types/factor_values.py	100.00% <100.00%> (ø)
formulaic/transforms/__init__.py	100.00% <100.00%> (ø)
formulaic/transforms/poly.py	100.00% <100.00%> (ø)
formulaic/utils/cast.py	100.00% <100.00%> (ø)

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