`BQM.into_file` or similar

Right now this can be done with
```
with open(.., 'wb') as f:
shutil.copyfileobj(bqm.to_file(), f)
```
but it would be nice to be able to just do
```
with open(.., 'wb') as f:
bqm.into_file(f)
```
and avoid the creation and copy of the intermediate file.