WIP: add Enzyme support for fastpow

[ChrisRackauckas]

Straightforward since fastpow is simply ^. Still needs:

• [ ] Tests
• [ ] Generalize to batchduplicated

[ChrisRackauckas]

@m-bossart here's some starter code.

[ChrisRackauckas]

One more step to finishing https://github.com/SciML/OrdinaryDiffEq.jl/pull/2282

[ChrisRackauckas]

@oscardssmith can you take a moment to clean up the downstream tests? Those seem to be due to some of your recent changes.