* Adds BorrowedAccount::check_sysvar().
* Adds BorrowedAccount::get_data_mut().
* Implements account resizing in BorrowedAccount.
* Exposes is_signer() and is_writable() in InstructionContext.
* Removes AccountMeta and get_instruction_accounts_metas().
* Makes throwing errors in BorrowedAccount optional.
* Removes result return values from BorrowedAccount.