StateTracker: Add a `Continuity` state tracker.

Authored by lepagevalleeemmanuel on Dec 11 2018, 5:46 AM.

Description

StateTracker: Add a Continuity state tracker.

This will eventually help reduce the need to loop all
StateTracker::Index to find "holes". For now it is used to help
reject invalid insertions. It will require StateTracker::Proximity
to be finished before being rolled into more code paths.

Details