Add lemma `set_fold_disj_union_strong`.

See merge request !267

Add a few lemmas

See merge request !266

Add function `map_kmap` that transforms the keys of a finite map.

See merge request !265

Thanks to @tchajed for pointing out the omission.

Various improvements to `Permutation` lemmas and instances

See merge request !270

Rename `Permutation_nil` and `Permutation_singleton` into `Permutation_nil_r` and `Permutation_singleton_r`.

Add lemmas `Permutation_nil_l` and `Permutation_singleton_l`.

Naming scheme: `operation_Permutation_{Proper,inj,inj_l,inj_r}`.

Comment about `EqDecision` in `Countable`.

See merge request !268

Thanks to @jules for pointing out these were missing.

Explicit visibility for Instances

See merge request !263