Signed and Unsigned message json impls

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Needed for json rpc api, allows json serialization by the serde tag or with the wrapper type
  • Obviously there is no perfect solution for the semantics about exposing these, but I think this is a good standard that is the most readable and usable. Please suggest if you think another way should be considered
  • I used the json from https://ipfs.io/ipfs/QmZqAv7Rj6rLv9EC5ho8S1JYYGZBnF3M8dnnsok2GzrSjU for test vectors of each

Reference issue to close (if applicable)

Closes #436

Other information and links