Associated constant generic as parameter of `StatefulHasher`

[mriise]

Not being able to have it as associate constant in current stable Rust sucks :-/

I guess we leave it like that then, until it;s possible in stable Rust? What do others think?

Originally posted by @vmx in #116 (comment)

[mriise]

Wait for const_generic to be more stabilized to support this

impl StatefulHasher for Hasher20 {
    const SIZE: usize;
    type Digest: Digest<Self::SIZE>;
}