(This is part of the initial work done in #863)

In order to commit both Bob and Ursula to each re-encryption request and response, we define the following format of "specifications" and "metadata". The specification is a formal description of the request, and it's sequentially signed by Bob and Ursula. The metadata is the result of these signatures, and it's included as an integral part of the re-encryption process (that's a feature of Umbral). The resulting CFrag, which contains the metadata, is signed by Ursula.

A more formal description follows. Let S_b(M) be the signature of M by Bob, and S_u(M) the signature by Ursula. Then:

specification = capsule || ursula_pubkey || alices_address || blockhash
task_signature = S_b(specification)
reenc_metadata = S_u(task_signature)

For each individual re-encryption request and response, the following dataflow applies:

 Bob			     Ursula
  |				|
  |				|
  |				|
  |  capsule, task_signature	|
  |---------------------------->|
  |				| reenc_metadata = S_u(task_signature)
  |				| cfrag = reencrypt(capsule, kfrag, metadata=reenc_metadata)
  |     cfrag, S_u(cfrag)	|
  |<----------------------------|
  |				|
  |				|

Define format for re-encryption metadata #259

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