Use associated types in const generics

[Pratyush]

Rust Tracking issue: rust-lang/rust#60551

This would allow use to simplify our Fp<P: FpConfig<N>, N> to just Fp<P: FpConfig> (as FpConfig would itself have N as an associated type. The same simplification would occur for our MontConfig and MontBackend.

[burdges]

It'll improve many things like avoiding this Vec but right now they've some odd problem with associated constants used internally not being recognized as appropriately bound: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2015&gist=e1114345b12197fdb922ef1acb2e80ae