Use gperf to generate a perfect hash table for keyword lookup. Adds a
python script which munges the output of gperf and adds additional
cleanup and optimisations.

Change-Id: I3656a7287dbd0688917893de3a671faef9e4578a
Reviewed-on: https://chromium-review.googlesource.com/c/1349240
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57790}