Interface of AD

[Axect]

In Peroxide 0.30, only up to 2nd order ADs are allowed. This is because of implemented structures. There are three kinds of available AD implementations :

1. Enum
   • Pros: Simple & Fast (on Stack)
   • Cons: For higher order AD, total size increases. (every members of enum has same size)
2. proc-macro
   • Pros: Fast & Effective
   • Cons: Heavy compile time (Can it be better in the future?)
3. Wrapper of Vec
   • Pros: Easy to implement higher order AD
   • Cons: Not effective (on Heap)

I used proc-macro until Peroxide 0.28, but I changed to Enum at 0.30. Enumis sufficient to calculate 2nd order univariate AD, but at higher order, it can be ineffective. (I'll check)
If we return toproc-macro, then it would guarantee best runtime performance. But it also provide worst compile time. If there is solution for reducing compile time, it will be the best choice.
And if we forgo our runtime performance, then we can get most convenient implementation - just wrapper of Vec.

Which do you think the best? Or if you have any other idea, then please let me know.