* Inline keyed_account_at_index() in all instructions of stake
which have more than one KeyedAccount parameter,
because these could cause a borrow collision.
* Uses transaction_context.get_key_of_account_at_index() in stake.
* Refactors stake::config::from to use BorrowedAccount instead of ReadableAccount.
* Replaces KeyedAccount by BorrowedAccount in stake.