Use const generics

[burdges]

We've min_const_generics approaching some stable release, which should suffice for replacing generic arrays everywhere.

Apologies if this came up elsewhere, not too plugged into these crates lately.

[newpavlov]

As written here min_const_generics are not sufficient for our needs. We also probably will need for rust-lang/rust#76560 to be resolved and stabilized as well (hopefully in a different form from the approach which the feature uses now).

[burdges]

I'll close this in favor of #238 then

[tarcieri]

We need min_const_generics + traits (i.e. using an associated constant as a const generic), so unfortunately the former alone is insufficient for our use cases.

There are a few places where we could consider const generics immediately. Namely in the der crate I use typenum (sans generic-array) to control the size of BigUint, but offhand that's really the only one I can think of where they're immediately applicable.