In most cases, either a &Bank or an Arc<Bank> is more proper.
- &Bank is used if the function only needs a momentary reference
- Arc<Bank> is used if the function needs its' own copy
This PR leaves several instances of &Arc<Bank> around; these instances
are situations where a clone may only happen conditionally.
The core/src/ directory is already pretty crowded, and moving these
items into the subdirectory more clearly identifies that they are tied
to banking_stage.