Use the Monoid structure of Endomorphisms to define powering

[Jacques Carette]

As was commented by @jamesmckinna , we really ought to use the definitions from Algebra.Definitions.RawMonoid for the RawMonoid structure on Endo, both the Setoid and Propositional versions.

[jamesmckinna]

See this commit based off #2342 for one approach: easily incorporated on top of what you have there!