gcodeInterpreter speedups ~30

I promise, this is my last one!

Roughly 30% speed increase from refactoring the parsing ot gcode information. On the RaspberryPi, the parse is very slow for files larger than a few hundred KB. This isn't a great improvement but it is all straight-forward and the code is just as easy to read. I don't think this can be pushed much further.

I've tested with Cura and Sli3r output on both the rPi and a x64 Linux box (where the speed increase means the parse takes 2.6s instead of 3.3s!).

I will say this isn't really a significant change, the Pi is still 'too slow for realtime' and a real PC is already 'fast enough'. If you don't think this is worthwhile just let me know and we can drop the pull request and I'll just revert my codebase.