Bool op refactoring

[adam-urbanczyk]

Will resolve #303

• Add __iter__ to Compound
• Refactor bool ops in shapes.py
• Refactor bool ops in cq.py
• Add tests

Additionally, this PR speeds up bool operation for some cases, e.g. repetitive patterns. It should at least partially solve #183 as well.

[codecov]

Codecov Report

Merging #312 into master will increase coverage by 0.09%.
The diff coverage is 94.59%.

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
+ Coverage   95.27%   95.36%   +0.09%     
==========================================
  Files          18       18              
  Lines        4636     4685      +49     
==========================================
+ Hits         4417     4468      +51     
+ Misses        219      217       -2     

Impacted Files	Coverage Δ
cadquery/cq.py	93.99% <86.66%> (+0.43%)	⬆️
cadquery/occ_impl/shapes.py	90.78% <94.73%> (+0.09%)	⬆️
tests/test_cadquery.py	99.20% <100.00%> (+0.01%)	⬆️

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[adam-urbanczyk]

I think it is ready for merging, please take a look @jmwright .

[jmwright]

@adam-urbanczyk Thanks!

I'm starting to get a little numb to codecov, which is unfortunate. It doesn't seem to evaluate the code coverage correctly and/or warns about changes that are so minuscule that they don't matter.