Consolidate predicate prepare storage slots function

[minghinmatthewlam]

Why this should be merged

In predicate_check and state_db we were duplicating code for preparing the predicate storage slots from a transaction's access list. This PR consolidates them to a util function to make sure they're in sync and reduces duplication.

How this works

• Creates separate file in utils predicate_slots.go
• Create utils PreparePredicateStorageSlots function
• Use util function for previous places.

How this was tested

n/a

How is this documented

comments in code

[aaronbuchwald]

LGTM - nice change, thanks for putting this up!