Idea: Add an option to set the mode to fs::copy

As mentioned by zackw, io::copy definitely can do those. The implementation literally first tries a function called kernel_copy and falls back to generic copying if that doesn’t work. Specifically on Linux the docs note that

On Linux (including Android), this function uses copy_file_range(2) , sendfile(2) or splice(2) syscalls to move data directly between file descriptors if possible.