[TCWG-614] Range extension Thunks

Worked all week on this. I've got a prototype that is nearly feature complete. It passes the existing tests when run in a single pass. I'm now trying to get it to run in multiple passes.

Plans [TCWG-614] Range extension Thunks Start adding tests for the new functionality and doubtless spending a lot of time fixing problems. If I'm lucky I may have something that I can at least in part send upstream for review by the end of the week.