add get_context_opt, refactor interpreter code
Large change. New bytecode system. Add retained frames
Fixes for new bytecode and memoized frames.
Large change. Refactor inlined state to be stack-based

 - State now uses retained frames instead of genenerated pack/unpack terms
Repairing unit tests after bytecode & state refactor
hashtable.toString: sort keys lexigraphically. Repair various tests.
Fixing test failures, revising how interpreter is invoked
WIP on change_events plan
Finish 1st draft of change_events.md