circuits: Add a way to construct a TxCircuit from bytes

[moCello]

Summary

Since the TxCircuit will replace the UnprovenTransaction struct in the rusk-prover we need a way to create a TxCircuit from a blob of bytes.

We can achieve this in two ways:

• Implement the Serializabla trait on TxCircuit to be able to create the struct from bytes.
• Implement a from_slice constructor

The implementation of Serializable would be cleaner, but since it would also need to be generic over the height of the notes tree and the amount of input-notes it would probably require an implementation with a macro which seems overkill for this purpose.

Relevant Context

See rusk #2069 and #229