elliptic-curve: hash-to-curve support

[kevinlewi]

It would be nice to have support for a from_uniform_bytes functionality for the elliptic curve implementations that performs the hash-to-curve operations described here, preferably the _SSWU_RO_ variants!

[burdges]

I'd expect some churn coming based upon https://mailarchive.ietf.org/arch/msg/cfrg/ZYlKNd4JJiJxNTYSwrY-ZFb4LeI/

[tarcieri]

I think we can potentially work on some hash-to-curve traits while the spec is still in flux.

Note that there is a FromDigest trait, however I have some misgivings about it: it's presently used for hash-to-scalar for the purposes of ECDSA, and as much, I've thought about moving it to the ecdsa crate for the next release. Edit: removed in elliptic-curve v0.9, clearing the way for new hash-to-curve traits