Helpers for JSON mailbox messages

[jml]

Lots of mailbox messages (i.e. the bytes in Messages.Body) are JSON encoded. The Magic Wormhole protocol documentation doesn't insist that this is the case, but all extant messages are JSON encoded.

There are some ways we can make this more convenient:

• A pair of ToBody and FromBody typeclasses

  class ToBody a where
      toBody :: a -> Messages.Body
  
  class FromBody a where
      fromBody :: Messages.Body -> Maybe a

• sendEncryptedJSON and receiveEncryptedJSON helpers

Probably other ways too.