Implement transaction balancing algorithm

[vlasin]

We need to design a custom balancing algorithm that

• chooses inputs,
• adds change outputs,
• and calculates fees
  All without running the Plonk prove algorithm as it is costly. The prove algorithm will run once the inputs and outputs are fixed and the fees have been calculated correctly.

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.