Various `omap` lemmas for finite maps; generalize `map_size_{insert,delete}`

* Add lemma `map_omap_union`.
* Add lemmas `map_disjoint_fmap` and `map_disjoint_omap`.
* Add lemmas `fmap_merge` and `omap_merge`.
* Add lemma `omap_delete`.
* Generalize `omap_insert` and `omap_singleton` to cover both the `Some` and `None` case. Add `_Some` and `_None` versions of the lemmas for the specific cases.
* Generalize `map_size_insert` and `map_size_delete` in the same way.
* Add lemmas `lookup_fmap_Some`, `lookup_omap_Some`, and `lookup_omap_id_Some`.