Total rework of simulator for dev branch. Create separate thread for interrupt processes. Tick-accurate simulation of timers. Non-blocking character input for running in realtime mode. Decouple hardware sim from grbl code as much as possible. Expanded command line options. Provisions for cross-platform solution.

sim/avr/io.c

@@ -0,0 +1,4 @@
+#include "io.h"
+
+// dummy register variables
+volatile io_sim_t io={{0}};