R1:f97157e7edb1

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.