Call floating wires "z" instead of "x"

[ausbin]

Blue wires are high impedance, and the standard name for that state is z, not x (e.g., in Verilog), so make CircuitSim display "z" instead of "x". Also remove the spots in the documentation where floating is called "uninitialized."

To me at least, this screenshot justifies this change:

Jokes aside, I want to use CircuitSim in lecture, and I don't want to explain that "x" in CircuitSim means "z," but in a K-map, "x" means don't care. I'm going to cherry-pick this directly into the 2110 fork, but I wanted to send this upstream to avoid us diverging too much.