[Cryptography] Schnorr multisignatures based on ED22519

[grarpamp]

There are real world in vivo applications of Schnorr
with lots of value riding on them that can always
be reviewed for correctness, curve selection,
multisig operations, implementation bugs, etc.

Here's one application example...

https://github.com/bitcoincashorg/bitcoincash.org/blob/master/spec/2019-05-15-schnorr.md
https://github.com/Bitcoin-ABC/bitcoin-abc
https://reviews.bitcoinabc.org/T527
https://reviews.bitcoinabc.org/maniphest/query/487xmasKpdn4
https://reviews.bitcoinabc.org/search/query/ChXUt.xy93mP
https://reviews.bitcoinabc.org/source/bitcoin-abc/browse/master/?grep=schnorr

https://github.com/Electron-Cash/Electron-Cash/pull/1331
https://github.com/gcash/bchd/pull/180

https://safecurves.cr.yp.to/
https://bitcoin.stackexchange.com/questions/34288/what-are-the-implications-of-schnorr-signatures