v3.0.0-beta.1

Breaking Changes

• feat!: queries return structs instead of tuples. Query methods have been updated so that they all return
  either a NearestNeighbour, Vec<NearestNeighbour>,
  or Vec<BestNeighbour>, for consistency.
• feat!: use a trait instead of a function pointer for distance metrics (See SquaredEuclidean and Manhattan)
• feat: add within_unsorted_iter query

Performance

• perf: refactor within to simply sort the result of within_unsorted.
  Previously, within was keeping its results in a BinaryHeap and calling
  its into_sorted_vec method to, well, return a sorted Vec.
  Whilst a BinaryHeap is great if you are frequently adding and removing
  items, if your use case is to gradually add all your items, and then sort
  them all at once, its quicker to just put things in a Vec and then
  sort the Vec at the end.
  Benchmarking shows that this change improves performance by anything from
  5 to 60% in practice.