- Pass Collector into TweakedScoreTopCollector and
CustomScoreTopCollector.
- Add std:: qualifier to f32, i32 etc. Not sure why this was not failing
already.
- Add unit tests for TopDocs with offset including for tweaked and
custom score collectors.
In order to convert a TopCollector<Score> to a TopCollector<TScore> I
had to add a `into_tscore` method to `TopCollector`. This is a hack but
I don't know how to avoid it.