Use ghost_map in proph_map and gen_heap

This starts to make use of the new ghost_map internally by porting proph_map and gen_heap to it. I'd say this is working rather well. :)

This is also a good opportunity to ensure that the API surface of ghost_map is consistent with gen_heap. To ensure that is the case, and for general convenience, I did some ghost_map changes:

• Rename ghost_map_elem_elem_frac_ne → ghost_map_elem_frac_ne, ghost_map_elem_elem_ne → ghost_map_elem_ne. (The gen_heap lemmas did not duplicate the mapsto either, so this seems more consistent.)
• Make ghost_map_elem_persist work for arbitrary dfrac.
• Add ghost_map_insert_persist.