Add type ExpandMsg to VoprfParameters

[daxpedda]

Main motivation was that decaf448 (RustCrypto/elliptic-curves#1121) requires ExpandMsgXof instead of ExpandMsgXmd.

I was briefly tempted to add a type Hash to the ExpandMsg trait to be able to write type ExpandMsg: ExpandMsg<Hash = Self::Hash>. Otherwise it is possible to add a different hash in the ExpandMsg type:

impl VoprfParamters for Fun128 {
    type Hash = Sha512;
    type ExpandMsg = ExpandMsgXmd<Sha256>;
    ...
}

But I thought its a bit much. Let me know if you want me to add that.