Add polygonize

[macdrevx]

Fixes #139

[macdrevx]

Code coverage will decrease slightly with this change b/c I couldn't find a way to exercise the error handling for polygonize.

[codecov]

Codecov Report

Merging #200 into master will decrease coverage by 0.05%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
- Coverage   94.81%   94.76%   -0.06%     
==========================================
  Files          46       46              
  Lines         810      821      +11     
==========================================
+ Hits          768      778      +10     
- Misses         42       43       +1

Impacted Files	Coverage Δ
GEOSwift/GEOS/GeometryConvertible+GEOS.swift	92.77% <90.9%> (-0.14%)	⬇️

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