Make use of improved halfedge removal helper (plus tidy and tests)

* tidy up remove-halfedge-helper
* tidy up and improve edge collapse - manually squash two sided faces (more concise than merge/remove)
* add test for valence-3 vertex merge (edge collapse)
* tidy up face merge and improve test
Complete mesh compaction and add native unused vertex cull
* add compaction helpers for faces and halfedges
* add public compaction method to mesh
* add cull unused to vertex list
* lots of tests!
Move FindHalfedge tests from vertex tests to halfedge tests