More lemmas for `filter` w.r.t. maps

See merge request !96

Fix Open/Close scope

See merge request !94

Use Open/Close Scope without Local (i.e., export the scope opening)
only when the scope corresponds to the main purpose of the module.

Due to Coq bug #10480 or #10474 it actually used `Morphisms.solve_proper`
instead of the version of std++. The version in std++ can inherently
not solve this, so I changed it into a manual proof.

Lemmas for fin_maps

See merge request iris/stdpp!91

Proofs about binders

See merge request !83

more instances for the empty type

See merge request !90

add Equiv instance for Empty_set

See merge request !89

Add `map_zip_with_flip`.

See merge request !88

relations.nsteps: add inversion lemma for `nsteps R 1 a b`

See merge request !87

Several simple lemmas.

See merge request !85

More lemmas about [map_imap].

See merge request !84

std++ 1.2.1 release notes

See merge request !86

some list lemmas

See merge request !82