Add instruction limit

[Timmmm]

Adds an optional --instructions=N CLI argument which will stop the simulation after N instructions.

This is useful for benchmarking and profiling and sometimes debugging.

[jerryz123]

I wonder if the desired behavior can be achieved through using the --debug-cmd flag, which lets you specify a file of interactive debug commands. Does setting the file to be

rs <some number of cycles>
q

achieve the same effect?

[Timmmm]

Doesn't seem to - it does stop the simulation but it just drops you into the REPL rather than exiting.

In any case it's a far less convenient and discoverable interface.