Consolidate more Ferret engine code into FerretEngine

Summary: Ref T12819. Earlier I separated some ngram code into an "ngram engine" hoping to share it across the simple Ngrams stuff and the full Ferret stuff, but they actually use slightly different rules. Just pull more of this stuff into FerretEngine to reduce the number of moving pieces and the amount of code duplication.

Test Plan: Searched for terms, rebuilt indexes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18533