Avoid relying on implicit instance generalization, and name some instances.

Merged Robbert Krebbers requested to merge robbert/implicit_names into master

Fix in preparation for https://github.com/coq/coq/pull/13188

I didn't name the Forall2 instances, since I could not come up with a good naming scheme. So, I just used the .

There are probably plenty of other unnamed instances, but I would prefer defer naming them.

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