Use ghost_map in proph_map and gen_heap

See merge request iris/iris!645

Flexible number of logical steps per physical step

See merge request iris/iris!595

* Rename ghost_map_elem_elem_frac_ne → ghost_map_elem_frac_ne,
  ghost_map_elem_elem_ne → ghost_map_elem_ne.
* Make ghost_map_elem_persist work for arbitrary `dfrac`.
* Add ghost_map_insert_persist.

add ghost_map library

Closes #358

See merge request iris/iris!562

Makes it possible to use several logical steps for one logical steps, in a way which can be controlled by ghost state.

Generalize frac to dfrac in auth and view camera

Closes #395

See merge request iris/iris!622

Fix typos

See merge request iris/iris!644

avoid relying on rewrite's implicit revert

See merge request iris/iris!643

finalize changelog for Iris 3.4 release

See merge request iris/iris!639

add better lemmas for working with mask-changing fupd, and rearrange names a bit

See merge request iris/iris!637