Expected changes to come:

* Collision avoidance
    * Slot Collider
    * Kern Collider
    * Associated glyph for exclusion
    * bounding octabox and subboxes
* FSM compression
* Setting segment feature values in action code, for pass constraint testing
* Move gralloc/realloc from once per rule to once per pass to speed up font loading.

Questions:
* Do we support glyphid as a slot attribute?
