See also Coq bug #5712.

Some lemmas about `difference` and `delete`

See merge request !5

Before, we often had to insert awkward casts when using them. Also,
the generality of also having them on Type, is probably not useful.

As suggested by Pierre-Marie Pédrot in the Coq-club thread:

  [Coq-Club] Very slow failing apply

To work arround some performance issues in Iris.

This is needed to use coq-stdpp in developments with -type-in-type
as set_unfold will otherwise unify with any hyp, causing the
set_solver tactic to break.

This way, we get more definitional equalities.