Method make_mutable

Makes the array mutable by possibly creating a deep copy of the data if it's either read-only or shared with another array. Stores the pointer to the beginning of mutable data in `data_out`. ``` BLArray a; if (a.append(0, 1, 2, 3, 4, 5, 6, 7) != BL_SUCCESS) { // Handle error condition. } uint8_t* data; if (a.make_mutable(&data) != BL_SUCCESS) { // Handle error condition. } // `data` is