Summary: Fixes T13472. This library uses `$a{0}`, but this is deprecated in favor of `$a[0]`.
Test Plan:
Ran `bin/search index Txxx --force` on a task with "filing" in the title (this term reaches the "m" rule of the stemmer).
(I'm not on new enough PHP for this to actually raise an error, but I'll follow up with the reporting user.)
Maniphest Tasks: T13472
Differential Revision: https://secure.phabricator.com/D20941
Summary: Ref T13472. Ref T13395. These classes are only used by Phabricator and not likely to find much use in Arcanist.
Test Plan: Grepped libphutil and Arcanist for removed symbols.
Maniphest Tasks: T13472, T13395
Differential Revision: https://secure.phabricator.com/D20939