Support arbitrary Blake3 sizes

[vmx]

In rust-multihash the digests are generic over their size. It should be possible to support arbitrarily sized Blake3 hashes. Currently the code will panic if you try a size differently from 256 bits.

[vmx]

Adding a different Blake3 size to the custom table example would be great.

[mriise]

#156 solves part of this, as the hasher Blake3<S> is exposed, though going through the table doesn't work as the proc macro expects a type without a generic.