Add BK Tree implementation

[maxbachmann]

It would make sense to add a BK Tree implementation for scorers which full fill the triangle inequality. This would provide massive performance improvements for things like searches.

https://dl.acm.org/doi/10.1145/362003.362025

[maxbachmann]

A common use case for things like search engines is to generate the tree once and then use it for searches. For this reason this should support some kind of serialization / deserialization. This should be simple for the tree structure itself. However it is unclear how this should handle the user specified data, since they could be in pretty much any data format. Probably this part should be left up to the user to do (strings could be serialized automatically).

[maxbachmann]

In the same context Levenshtein automatons could be interesting as well: https://dmice.ohsu.edu/bedricks/courses/cs655/pdf/readings/2002_Schulz.pdf