As Protobuf is going to support Cord to reduce memory copy when
[de]serializing Cord fields, gRPC is going to leverage it. This
implementation is based on the internal one but it's slightly modified
to use the public APIs of Cord. only
* Clean up `grpc_byte_buffer_copy()`
* Clean up `grpc_byte_buffer_destroy()`
* Clean up `grpc_byte_buffer_length()`
* Clean up `grpc_byte_buffer_reader_init()`
* Clean up `grpc_byte_buffer_reader_destroy()`
* Clean up `grpc_byte_buffer_reader_next()`
* Clean up `grpc_byte_buffer_reader_peek()`
* Clean up `grpc_raw_byte_buffer_create()`
* Clean up `grpc_slice_new_with_user_data()`
* Clean up `grpc_slice_new_with_len()`
* Clean up `grpc_call_start_batch()`
* Clean up `grpc_call_cancel_with_status()`
* Clean up `grpc_call_failed_before_recv_message()`
* Clean up `grpc_call_ref()`
* Clean up `grpc_call_unref()`
* Clean up `grpc_call_error_to_string()`
* Fix typos
* Automated change: Fix sanity tests
Esun Kim