Summary:
This change refactors the Engine to use a different
code path for when we're calling `lookupItem` to find
a first token `Node` matching the rule and a different
one for subsequent ones.
This division lets us get better invariants and more importantly
do full text regexp matches only when necessary.
This should be particularly useful for longer texts.
Reviewed By: patapizza
Differential Revision: D4953918
fbshipit-source-id: e3a69ad