Point::steg_encode was leaving the 24 high bits of the buffer as zero.
It also ignored the size parameter.  The size parameter has now been removed,
the zeros fixed and a test added to make sure that it is fixed.

Per https://github.com/MystenLabs/ed25519-unsafe-libs, deprecate eddsa signing
with separate pubkey and privkey input.  Instead decaf_ed*_keypair_sign