Rewrite cross split lemmas so they can more easily be used for forward reasoning.

See merge request !283

Various setoids lemmas for maps, lists, and option

See merge request iris/stdpp!281

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