elliptic-curve: Allow multiple dsts in the hash2curve API

[daxpedda]

After #1175 got merged I attempted to go ahead and update voprf and opaque-ke, but quickly hit the same problem that required #876.

The change to the OPRF ID now being a string instead of a byte made it impossible to generate a fixed size array to hold the DST. So I decided to apply the same solution I did in #876.

There are only two changes:

• All hash2curve APIs now take &[&[u8]] instead of &[u8] as the dst parameter.
• ExpandMsgXmd now requires Default + FixedOutput + HashMarker instead of Digest.

[tarcieri]

@daxpedda if you can update https://github.com/RustCrypto/elliptic-curves that'd be great. It's pulling in elliptic-curve from git again

[daxpedda]

On it.