Borrowed Accounts API Rewrite

[cali-jumptrading]

Rewrite of borrowed accounts to make code cleaner and simpler.
Tasks:

• Consolidate Borrowed Account API checks. All other wrapper borrowed account APIs (in instr context and txn context) should use the same borrowed account API. Borrowed Accounts API Consolidation #4306
• Ensure that we're not missing any checks in the account access apis that Agave has
• Simplify storage of account pubkeys and borrowed account pointers in instruction context. We should just store indices into the transaction context.
• use function vtable to make borrowed accounts behave more like a class?
• more stuff coming...

[topointon-jump]

• Ensure that we're not missing any checks in the account access api's that Agave has