Add some type annotations for prepare for `Hint Mode` for `Equiv`.

Merged Robbert Krebbers requested to merge robbert/type_annotations into master

I tried to set the hint mode for equiv:

Global Hint Mode Equiv ! : typeclass_instances.

Due to a recently fixed Coq bug, this only works with Coq 8.15+. So, while we cannot merge the hint mode yet, this MR contains some missing type annotations I discovered in the process.

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