71B: DISP, KEY$, and keyboard buffer

[rprosperi]

Since the intended use is to ultimately resume execution after a long wait, you should strongly re-consider replacing KEY$ with KEYWAIT$, which puts the 71B into a sleep-like, low-power mode until a key is hit. KEY$ does appear to similarly just wait, but it's really running a near full program execution level inside (probably just looping and checking the key buffer), so if the calculations run for 30 secs and then you don't notice it has been waiting for another 30 secs, it's about the same as if the calculations ran for the full minute. KEY$ eats batteries for sure.