Address advice witness redundancy

[igamigo]

The TransactionWitness struct currently contains some redundancy which should be minimized.

In order to do so, @bobbinth suggests the following:

• All advice-related data could go into the advice_witness field. This could include everything that would be needed for executing the transaction including things like TransactionArgs.advice_inputs, note args, some data from the foreign account inputs etc.
• All code-related data could go into a single field which could be a vector of MastForests (or maybe even a single merged MAST forest).

Basically, TransactionWitness would contain some metadata about the transaction, but the data that needs to be go into the advice provider and MAST forest store would be stored separately.

(Reference comment)