Fixed duplicate edge handling

[elalish]

Fixes #176

In pairing halfedges to construct the manifold, in the case of a duplicate edge (not a 2-manifold, which is occasionally caused by the triangulation), it was possible that I would pair the two forward edges together and the two backward edges together, which doesn't work. Now I've adjusted the way I sort to ensure I always pair a forward edge with a backward edge.

I've also separated the internal IsManifold check into two: IsManifold now only checks for data structure sanity (the problem we had here, and something that should be true at every step of the algorithm), while the new Is2Manifold also checks for edge duplication. The public IsManifold's behavior is unchanged. I've added asserts for IsManifold to catch this kind of problem in the future more quickly, but they only get triggered in verbose mode.

[codecov-commenter]

Codecov Report

Merging #179 (6c0fc3f) into master (bf2a9a8) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
+ Coverage   96.53%   96.57%   +0.04%     
==========================================
  Files          30       30              
  Lines        2624     2630       +6     
==========================================
+ Hits         2533     2540       +7     
+ Misses         91       90       -1     

Impacted Files	Coverage Δ
src/manifold/src/impl.h	100.00% <ø> (ø)
src/manifold/src/boolean_result.cpp	100.00% <100.00%> (ø)
src/manifold/src/impl.cpp	99.47% <100.00%> (+<0.01%)	⬆️
src/manifold/src/manifold.cpp	89.65% <100.00%> (+0.68%)	⬆️
src/manifold/src/properties.cpp	90.90% <100.00%> (+0.18%)	⬆️

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