Not clear which part of the bank is the runtime

[garious]

Problem:

We talk about the "runtime" in the contracts engine RFC, but it's not in a module or a struct of its own. It's some set of Bank methods acting on a subset of Bank state. Without that separation, it'll be difficult to ask others to scrutinize that mission-critical code.

Proposed solution:

Split the bank. execute_and_commit_transactions() everything it calls to a new runtime.rs module.

Depends on #1529

[garious]

execute_and_commit_transactions(), as previously proposed, might be too much for runtime.rs. Instead, only execute_transaction() looks useful on its own. A client could use it to test their transactions and then assume ACID when submitting those same transactions to a Solana cluster.