Build a prototype fulltext engine ("Ferret") using only basic MySQL primitives

Summary:
Ref T12819. I gave this stuff a sweet code name because all the terms related to "fulltext" and "search" already mean 5 different things. It, uh, ferrets out documents for you?

I'm building this to work a lot like the existing ngram index, which seems to work pretty well. If this sticks, it will auto-resolve the join issue (in T12443) by letting us do the entire thing locally in a JOIN and thus dodge a lot of mess.

This index gets built alongside other indexes, but only shows up in the UI if you have prototypes enabled. If you do, it appears under the existing fulltext field in Maniphest. No existing functionality is affected or disrupted.

Test Plan:
Indexed my tasks, ran some simple queries, got the results I wanted, even for queries "ko", "k", "v0.1".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819, T12443

Differential Revision: https://secure.phabricator.com/D18484