This proof also more easily scales to other recursive types, like
trees etc.

Notably, `big_andL_andL` and `big_andL_and` where a ⊣⊢ and ⊢ version
of the same lemma. I favored the `big_opL_op` naming scheme.

This comment wasn't updated after
866cad62.

This is in preparation for coq/coq#9274.

This lemma allows one to get the witness out of a later, without having
to use `later_car`, i.e. in a way that works in the on-paper version of
the logic.

Adding a hint without a database now triggers a deprecation warning in
Coq master (https://github.com/coq/coq/pull/8987).

Added a new lemma to allocate fragmented ownership in unital gmaps while updating the full ownership