* Make TopCollector generic
Make TopCollector take a generic type instead of only being tied to
score. This will allow for sharing code between a TopCollector that
sorts results by Score and a TopCollector that sorts documents by a fast
field. This commit makes no functional changes to TopCollector.
* Add TopFieldCollector and TopScoreCollector
Create two new collectors that use the refactored TopCollector.
TopFieldCollector has the same functionality that TopCollector
originally had. TopFieldCollector allows for sorting results by a given
fast field. Closestantivy-search/tantivy#388
* Make TopCollector private
Make TopCollector package private and export TopFieldCollector as
TopCollector to maintain backwards compatibility. Mark TopCollector
as deprecated to encourage use of the non-aliased TopFieldCollector.
Remove Collector implementation for TopCollector since it is not longer
used.