Bump Tantivy to version 0.20 and adjust the code base to match

While the index size is unchanged at 43MB and the included benchmark without spatial filtering improves from 40ms to 20ms, the performance when accessing the bytes fast field storing the bounding boxes regresses 20-fold from 40ms to 920ms.

Hopefully, this is just an error in how the filter collector is adjusted. Still trying to figure this out...

Closes #208 (closed)

Closes #209 (closed)