Switch everything over to the Ristretto variant of the Decaf encoding,
for compatibility with https://github.com/isislovecruft/curve25519-dalek
and others.  Documentation coming sometime at https://ristretto.group/.

For now, suffice it to say that Ristretto has the same API and security
properties, but slightly different input and output bytes.

Also close Ticket 9, renaming "mul_by_cofactor_and_encode" functions to
"mul_by_ratio_and_encode" with an appropriate #defined ENCODE_RATIO.
This is because the base points in RFC 7748 and RFC 8032 have different
relationships with the libdecaf base points.  With this change,
mul_by_ratio_and_encode'ing the base point will result in
[EDDSA|X25519|X448]_ENCODE_RATIO times the RFC 8032 EdDSA and RFC 7748
X25519|X448 base point.

Merge branch 'ristretto-work'