Refactor levels, key range checking etc

[marvin-j97]

Currently, all the level logic operates basically on a Vec<Vec>, which can be a pain to unit test because we have to create fixture segments.

It would be better if all the key range checking, overlapping in other levels can be utility functions over a trait K that is basically:

trait K {
  fn id(&self) -> Id;
  fn key_range(&self) -> &KeyRange;
  fn size(&self) -> u64;
}

and Segment implements that trait (quite trivially).

And then have utility functions for &[K] like:

fn overlaps(&self, key_range: &KeyRange, hidden_set: &HashSet<Self::Id>) -> impl Iterator<Item = K>;

fn find_by_key(&self, key: &[u8]) -> Option<&K>;

// ...etc

That way, all the range checking, overlapping logic etc is separated from segments.