P384 support

[raphaelrobert]

Since P384 is mandated as the only valid ciphersuite by Privacy Pass, it would be nice to have support for it.

I suppose it is blocked by RustCrypto/traits#1024.

[daxpedda]

I can attempt to push this forward in a different way so we wouldn't have to wait for the next version of elliptic-curve if this is blocking you.

[daxpedda]

With #101 merged the p384 crate is now supported.
#84 will add tests to voprf.

Currently neither voprf or p384 have a crates.io release for P-384 support though.