gofmt of nodeprep.go + MUCH faster random unicode generation

Using constants from unicode/utf8 to find runes that would be considered valid according to http://golang.org/src/pkg/unicode/utf8/utf8.go?s=10311:10338#L415

But the python component seems to be broken. It doesn't accept valid UTF8.

If the python lib we use is the way Spotify does it, I suspect we will never be able to do it the exact same way without using the exact same code... how can we reproduce all the bugs they have?