R1:0e2e525bb41a

Add a "terms" corpus to Ferret fields

Summary:
Ref T12819. Ferret currently does substring search, but this is not the default mode users expect: when you search for the "RICO" act, you do not expect to find documents containing "apRICOt" even though "RICO" is a substring.

To support term search, index the corpus as a list of terms with puncutation removed and whitespace normalized so the engine can match against it.

Test Plan:
Ran `storage upgrade`, ran `search index`, saw sensible database results…