Use SourmashSignature for top-level search and Index APIs.

[ctb]

in re #1370,

we should make sure that the top-level APIs for search and Index are using SourmashSignature and not MinHash objects. That protects us from specific details of the sketch, and potentially lets us make use of fine tuned, or more advanced, or just plain different sketch types.

Right now most of the top-level APIs do use query rather than query_mh, but the peek and consume methods and the make_gather_query function take MinHash directly, and peek returns a MinHash as well.

ref #616