ECKey: remove *Hex/*Buffer operations

These operations are not complete in their serialization of the
underlying private key; as they miss compression information that is
pertinent to the matching public key.

toWIF/fromWIF should always be preferred.

If serialization of the raw private key is necessary, it is simple
enough to simply do: `priv.D.toBuffer()`.
So this is not a regression in convenience or functionality, just a
stricter subset of overall functionality.
message: add network-based Magic prefix