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

The existing code checked if a directory existed and if it did, it would
not attempt to create it. However, this proved to be flaky on Mac and Linux.
The check would frequently report the directory as not existing, but the
attempt to create the directory would cause an exception.