Use const generics

[rklaehn]

They are finally there: https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html

Should help a lot with the very verbose types we currently use...

[mriise]

would changing the API to look more like

Multihash::BLAKE_3<SIZE_BYTES>

be the right move for this or do we keep tables the way they are, and maybe just add this on top of what we have @vmx

I'm almost finished with a draft using const generics for the backend of things, just would need to change the beast that is the proc macro.

If we have this as just hidden behind a feature for now, I could ignore trying to mess with that and just go with something completely based off of using const generics

[vmx]

I haven't thought about this yet. My intuition is that I probably would keep things as they are and just doing a move to const generics. A next step would then be to improve/change the API if it makes sense. Though as I haven't looked at it, it might also make sense to do it at once.

@mriise If you're happy to take comments, feel free to PR whatever you currently have and then we can discuss details on the PR.