Some renaming and cleanup of the Input, Output, Transaction and
Block structs. The main change is the removal of all the overt
structures which are now replaced by a specialized module to
build transactions easily. More specifically:
* Rename the TxProof to TxKernel in Block to reflect the current
naming consensus in MimbleWimble.
* Change Input and Output to be plain structs instead of enums
making their manipulation a lot easier. The building of
transactions is now handled by the build module.
* Input now directly includes the commitment of the Output it's
spending instead of going through an intermediate hash.
* The new build module encapsulates all the transaction building
logic, making it very straightforward by chaining combinators.
* Moves some tests to the core mod.rs as they required being able
to build a transaction to test it.
* use MESSAGE_SIZE constant instead of 32
* use Hashed trait in genesis
* safer serialization of Hash
* Hashed of [u8] should send only data to hash function, not length + data
* Safer serialization of Proof
* Safer serialization of Commitment
* Safer serialization of RangeProof
* introduce read_limited_vec instead of potential panic in conversion
