Replace PROCESS's "command stack" with a program counter

[ecpeterson]

It is unrealistic from the perspective of hardware to keep a full list of commands-to-be-executed for a running process. Rather, processors tend to carry a fixed set of instructions, a pointer to the current instruction step, as well as a list of pointers to past callsites to which the processor intends to return. This wildly reduces memory complexity, at the awkward expense of having to have a fixed "code segment" in memory.

We will ultimately move toward this model as we begin microarchitecture emulation. It is at least worth keeping this in mind as we work now, but ultimately this will turn into a proper task of enabling this kind of "program counter" tracking.