Account borrow state check

[febo]

Problem

The account borrow check done on CPIs is slightly more expensive than it needs to be, since it also checks whether another borrow can be made or not.

Solution

Create a BorrowState enum to represent different borrow masks. This allows to perform checks on the account borrow state. This will enable CPI helpers that already set the accounts in the correct order to use *_unchecked variants, but still perform borrow checks.

It also makes 2 other improvements related to CPIs:

• inlines the invoke_signed_unchecked
• uses a MaybeUnit for the pubkey in the get_return_data

[joncinque]

This looks really great! Just a couple of nits