* Makes error handling in BorrowedAccount optional.
Adds BorrowedAccount ::get_rent_epoch().
Exposes InstructionContext::get_index_in_transaction().
Turns accounts and account_keys into pinned boxed slices.
* Introduces "unsafe" to InvokeContext::push().
* Turns &TransactionContext into &mut TransactionContext in InvokeContext.
* Push and pop InstructionContext in InvokeContext.
Makes test_process_cross_program and test_native_invoke symmetric.
Removes the borrow check from test_invoke_context_verify.
* Removes keyed_accounts from prepare_instruction()
* Removes usage of invoke_stack.
* Removes keyed_accounts from program-test.
* Removes caller_write_privileges.
* Removes keyed_accounts from BPF parameter (de-)serialization.
* Adds TransactionContext, InstructionContext and BorrowedAccount.
* Redirects the usage of accounts in InvokeContext through TransactionContext.
Also use the types declared in transaction_context.rs everywhere.
* Adjusts all affected tests.
Alexander Meißner