Finally bind C-n to something analogous to 'new document'.

This was pretty much the last binding that didn't do what you'd expect
it to do based on every other editor in the history of ever. It
doesn't quite create a new document every time you type it, but if you
save & close your *scratch* buffer, C-n will now create a new one for
you, so you can at least have one new untitled document at a time.