EnzymeRules

[michel2323]

This adds EnzymeRules to the supported rules system.

• There's a new folder, src/Rules, with ChainRules.jl and EnzymeRules.jl checkpointing rules.
• Removed all deprecated code, including dependencies on other AD tools besides Enzyme, which we need to differentiate the checkpointed loop. Only having Enzyme for that until another mutation-supporting AD tool comes out is okay.
• Any AD tool supported by one of the two rule systems should work outside the checkpointed loop.
• I took note of other stuff I want to refactor/change. I leave that for later minor releases.
• All tests on the examples are done using both Zygote and Enzyme on the code outside of the checkpointed loop.

[vchuravy]

Looks great! Can we try disabling the GC on/off?

[michel2323]

Looks great! Can we try disabling the GC on/off?

Yea. It's on my list. I'll put in an option.