Welder: Loops, Functions, Conditionals

Welder's getting more powerful by the hour.  I've now got looping, basic function calling (replacing what I used to call "groups"), and conditional branching.

Here's another dumb test scene:

And here's the code:

I've still got to implement functions in expressions and argument passing.  There's also a small bug in parsing a certain type of expression, but I think I know how to fix it.  Finally, I'd like to implement unary negation, since writing 0-x is rather annoying...but I'm not sure it's worth the effort.  After I nail those down, most of the core language should be done. The rest will just involve adding more powerful intrinsics.