It would be handy to be able to pass the `length` default argument defined by `shutil.copyfileobj()` to other public functions througout the module. When copying very large files (1GB +), increasing the memory buffer can divide the copy time in half and currently the only way to accomplish this is by overriding the needed function(s) which call `shutil.copyfileobj()`.

I propose a simple non-invasive change where the following functions will also expose the `length` kwarg and pass it downwards to `copyfileobj`:
- copyfile
- copy
- copy2