What Sort module symbols should the Search module reexport?

[jabraham17]

Currently the Search module reexports the two provided comparators by Sort, DefaultComparator and ReverseComparator. This is done so that users of Search can make use of the same comparators.

The old Sort module API (prior to Chapel 2.2) allowed users to define their own comparators through ducktyping/magic methods, but the new API requires a custom comparator to implement one of three interfaces. Presumably, the Search module will take a similar approach. So it may be useful for the Search module to also reexport the comparator interfaces (keyComparator, keyPartComparator, relativeComparator, and sortComparator).