* Replaces KeyedAccount in create_executor().
* Refactors len to write_offset in write_program_data().
* Replaces KeyedAccount in write_program_data().
* Use transaction_context.get_key_of_account_at_index() in process_instruction_common().
* Renames next_first_instruction_account to program_account_index.
* Replaces program KeyedAccount by BorrowedAccount in process_instruction_common().
* Removes _program in process_instruction_common().
* Replaces first_account KeyedAccount by BorrowedAccount in process_instruction_common().
* Moves KeyedAccount lookup inside common_close_account().
* Replaces close_account, recipient_account and authority_account KeyedAccount by BorrowedAccount in common_close_account().
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