Add a localized emulation testbed

[jverkoey]

Selecting a function or range of instructions should allow the selection to be extracted into a testbed where the input register and memory values could be played with in realtime as the values are passed through the instructions, with the results shown at the end. This would work similar in principle to Swift playgrounds, where values are shown in real time alongside each line.

The intent of this testbed is to enable an easier sense of what a body of code's purpose is.