What does Execution#clone do about locals?

[ELLIOTTCABLE]

There's been discussion of some sort of forking-data-structure as the locals type, which obviously adds untold complexity to the language … but failing that, we either:

• Copy all of the locals-pairs (thus, variable definitions) to a new locals, thus leaving the new fork of program execution indicated by the new Execution distinct from any other forks,
• Share the exact same locals object.

Both have upsides and downsides: the former leaves any execution that gets called (because call-pattern clones; including those that co-consume), no longer sharing locals with any other executions of the same Script, which may be rather-less-than-intuitive. :P

(The latter, obviously, also means needing to work some implicit ownership-management shit into locals accesses. Not sure exactly what to do about that, could get messy quick.)