elliptic-curve: extract `hash2curve` into its own crate?

[tarcieri]

I was able to obtain ownership of the hash2curve crate from @mikelodder7.

I would propose extracting the hash2curve module out of the elliptic-curve and into its own crate, placing it in the https://github.com/RustCrypto/elliptic-curves.

Though hash2curve defines a lot of traits it's also an algorithm implementation. I think it would also be easier to experiment with changes to how it works (e.g. #1925) if it were in-tree with the various elliptic curves.

cc @daxpedda @carloskiki

[daxpedda]

Sounds great!
I will probably get to it in the next couple of days.