This task unblocks issue # 6.

This algorithm prototype:

only chooses inputs (disregards reference inputs and validity range)
adds change outputs,
and calculates fees
Runs the Plonk prove algorithm for the (cheap) IdentityCircuit.

Inputs to the algorithm:

Partial transaction to be balanced;
A set of input UTxOs that could be consumed to balance the transaction;
The address for change outputs.
The algorithm outputs the balanced transaction ready to be submitted to the blockchain.