Reduce the number of magic strings in the Ferret implementation

Summary:
Ref T12819. Push more of the magic ' ' stuff into the engine and simplify calls to ngram construction.

Also fixes a bug where a task with title "apple banana" and description "cherry doughnut" could match query "banana cherry" by separating separate term segments with newlines instead of spaces.

Test Plan:

• Indexed some objects.
• Searched (term, substring, quoted terms).
• Viewed index in database.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18534