Draft: WIP: Proto, RPC, and Client: support for the TZT format

Client support for the TZT format proposed in tezos/tezos!1487 (merged).

For simplicity, the step function of the interpreter is exported as an RPC but since TZT tests are supposed to be context independent it is not strictly needed and this could be achieved without upgrading the protocol.

Some features are still missing. Some of them (like overriding of NOW) would probably be easier if we chose to go the free-monadic way.

TODO:

• Implement all the fields of the TZT format
  • amount
  • balance
  • chain_id
  • now (by adding an interpreter step constant)
  • source
  • sender
  • big_maps
  • other_contracts
  • parameter
  • self
• Parse outputs to support the types that have several representations (such as timestamps for example)
• Wildcards in output stacks
• Support for runtime failures