Define TIntermTraverser and TIntermLValueTrackingTraverser in a
separate header file. hash() function is moved out from
TIntermTraverser as it is not related to the core functionality
of traversing and transforming ASTs.

Also reorganize some traversers to follow common conventions:

- Intermediate output is now in OutputTree.h/.cpp
- Max tree depth check is now in IsASTDepthBelowLimit.h/.cpp

BUG=angleproject:1490
TEST=angle_unittests

Change-Id: Id4968aa9d4e24d0c5bac90dc147fc9f310de0184
Reviewed-on: https://chromium-review.googlesource.com/559531Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>