Replace pc/stack with dynamic Script

[ELLIOTTCABLE]

At the moment, we've got a Script that acts as a static representation of combinations: an Execution maintains a pointer into that, of some sort. The Script doesn't change, and as the Execution dives into the combinations, it maintains a ->stack attribute of combinations-in-progress.

The alternative: copy the Script for each clone of the Execution, store it as a part of the data-type, then as the Execution is realized, we gradually replace nodes with the results of their respective combination:

L'foo' L'bar' L'baz'
O<foo> L'bar' L'baz'
O<bar> L'baz'
O<baz>

This precludes the necessity of a stack entirely. However, implementationally, it'd require a lot more copying.

(Also, this is kinda implementation-details-y. But, at the moment, the spec is an ugly hybrid of implementation-details and operational-semantics. So, an easier-way-to-understand-it description is the purpose of this issue.)