Got the walker API to work, for safely recursing the mesh.

Got the walker API to work, for safely recursing the mesh.
Used it to implement the dissolve faces operation (previous
incarnation was just a debugging hack).  The code works by
creating one giant new face per region of faces.

The dissolve verts (xkey->collapse, heh need to rename it)
operator now invokes dissolve faces on the faces around verts.
This is less error-prone then a pure topological/euler based
solution.