feat: UTF-8 and base256emoji

[Jorropo]

Related: multiformats/multibase#88 (note it's not actually blocking because my last commit already update to the commit on my branch).

This include fixes for UTF-8 as well as base256emoji encoding (an encoding which actually use UTF-8).

If you want to know why I think this should be merged, see multiformats/multibase#88. :)

[BigLep]

2022-01-07: we're interested in including this because it shows we truly can support base 256 (and it's fun).

I left some comments on characters that aren't visible on macOS 12.0.1.

@rvagg : not a high priority at all, but do you have any concerns with this getting merged?

[rvagg]

This is fine by me, but it'd be good to get multiformats/multibase#88 sorted first, which Steven had some comments on re validity - which maybe should be coupled with the feedback here about FF availability - ensure we get the proper set included. Once we get it sorted maybe I'll try and put a version in js-multiformats too for fun.

[Jorropo]

Updated to the new charset which includes comments made.

[Jorropo]

Note, in case multiformats/multibase#88 isn't merged with merge strategy (but with squash or rebase).
It is important to update the submodule.

[willscott]

Use of utf8 methods looks reasonable

[rvagg]

thanks for the input Will