Remove mesh._cells and mesh._inv_cells

No longer used anywhere.
Implement fast determination of old_to_new cell maps

We need these to back out the coarse cell to fine dof maps in the
multigrid hierarchy.  So implement in Cython and only build on demand in
MeshHierarchy construction.
Rework get_cell_renumbering to get_entity_renumbering

We need the vertex permutation too.