Probably not related to this particular PR, it occurs to me that ecdsa maybe not be the best place to host `parseSigCompact` because

1. It is not used else where other than message sig verification. And I don't know how useful it is to expose it as a publicly accessible API.
2. Sitting side by side with `parseSig` it is just confusing.

Maybe consider moving it into message.js and make it a internal function?