Begin port of edge subdivide (with multicut) to

Begin port of edge subdivide (with multicut) to
bmesh.  Basic infrastructure is in place, and
subdivision patterns for quads are implemented.
Properly handling of the (many) flags and options
related to edge subdivide isn't complete, but I've
made a good start.

Goal is eventual 100% api compatibility with old
esubdivide function, with some of the uglier design
aspects refactored after some of the tools that rely
on them are redone.

Some notes: Customdata interpolation wasn't working
right.  I thought maybe the weights were off, so I
swapped them, which surprising made it work.  It's
still not completely identical to old edge
subdivide though.