Add lemmas `map_intersection_filter` and `map_difference_filter`.

See merge request iris/stdpp!282

Prove more equivalences for closure operators on relations.

See merge request iris/stdpp!278

As part of this: turn `rtc_nsteps` and `rtc_bsteps` into ``s.

The `_list` lemmas were proposed by @jules and he provided an initial proof specific to `rtc`.

Misc lemmas for maps

See merge request iris/stdpp!280

Misc improvements to `head` and `tail` functions for lists

See merge request iris/stdpp!277

Drop `DiagNone` precondition of `lookup_merge` rule of `FinMap` interface.

Closes #94

See merge request iris/stdpp!279

Misc setoids lemmas and tweaks for maps and option

See merge request iris/stdpp!276

Add `Proper`s for maps, and generalise existing ones. Add tests to check that the old ones can be derived.

Rename `option_mbind_proper` → `option_bind_proper` and `option_mjoin_proper` → `option_join_proper`.

Make filter lemmas for maps and sets consistent + add cross split property for maps

See merge request iris/stdpp!274