feat: add proquint multibase

[rvagg]

I've got fairly low confidence in this, but I had fun doing it.

From what I can see, reference implementations depend on being able to read inputs as 8-bit pairs (i.e. encode and decode in 16-bit chunks), but we need more flexibility than that for multibase so I've taken some liberties in defining what an uneven number of input bytes would do, which you can see in the test fixtures, i.e. instead of a 5 character block, it's a 3 character block, so we overspend by 2 bits.

Need to find other implementations that deal in variable input and output bytes and not just strict 16 or 32 bit ints.

Anyone want to help!?

[rvagg]

Ref: https://github.com/multiformats/multibase/blob/master/rfcs/Proquint.md

(for the pro- prefix, not just p)