Add in a sortable base64 encoding from multibase

[CMCDragonkai]

Is your feature request related to a problem? Please describe.

From this issue multiformats/js-multiformats#124, it is clear that the highest sortable encoding right now in multibase is base32hex.

When using this for IdSortable, from 16 bytes, we get a encoded string length of 27 (v00qnlvl084e15mk9inn1en2gno). A higher base like base64 would be nicer as it could give us 23 characters instead.

There are 2 implementations of sortable base64.

• https://www.codeproject.com/Articles/5165340/Sortable-Base64-Encoding
• https://github.com/deanlandolt/base64-lex
• https://github.com/dominictarr/d64

Obviously what we would like is something that looks like base58btc as it is intended that such ids would be presented to the end user, and but get a guarantee that it is sortable.

Describe the solution you'd like

A human-centered readable encoding for IdSortable that is base64 (it has to be double of base32), that is sortable.

There's no existing standard for this except uuencode according to: https://stackoverflow.com/questions/53301280/does-base64-encoding-preserve-alphabetical-ordering#comment101660912_55011262

Describe alternatives you've considered

• base32hex is the only viable one atm

Additional context

Expect this to be used by notifications, sigchain and all the places we use IdSortable.

[CMCDragonkai]

@tegefaulkes this means base32hex needs to be used everywhere IdSortable is used atm.