Make iSpecialize work with coercions.

For example, when having `"H" : ∀ x : Z, P x`, using
`iSpecialize ("H" $! (0:nat))` now works. We do this by first
resolving the `IntoForall` type class, and then instantiating
the quantifier.
1 job for master in 53 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
canceled iris-coq8.6 #2601
coq

00:00:53