simplify: Improve edge generation/sorting performance
simplify: Improve edge generation/sorting performance

This change improves simplification performance by reducing the size of
Collapse struct from 24 to 12 bytes on 64-bit platforms and by sharing
the memory for temporary objects between simplification passes (which
reduces allocations and page fault count).

On buddha.obj, this change reduces simplification time from 7.5s to 6.5s.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer