Implement SST function properly

[ghewgill]

From Barry Mead:

I also noticed that the SST button on your simulator does not work the same as
the SST button on the REAL HP-15C. When you press the SST button on the real
calculator, it shows you the program step and instruction in calculator
button coordinates. When you release the SST button it shows you the result
of executing that instruction. This turns out to be very useful in testing
and diagnosing HP-15C calculator programs. If you know where you are in the
program, then you can tell if the program is doing what is expected at each
step. The way it works now you are kind of flying blind when you press the
SST button, since you cannot tell which program step you are executing or
what the instruction is at that step.