Function parser for OpenSCAD and parametric curve grapher

One of the OpenSCAD limitations is the inability to pass functions to functions or modules.
This library lets you define mathematical functions in a string, using all of the OpenSCAD mathematical operators (and a bit more). This string is then compiled to an optimized representation which can be evaluated fairly quickly. For instance:

use ;
f = "pow(x,3)*y-x*cos(y)+y";
fc = compileFunction(f);
echo(eval(fc, [["x", 12], ["y", -5]]));

will evaluate the function at x=12 and y=-5.

Parsing the string is not fast, but if you evaluate the compiled function for different variable values, the evaluated function should be fairly fast (in my test, about 10,000 `eval()`s per second).

There are three public functions provided:
* `compileFunction(f)`: takes a function string `f` and returns an optimized compiled representation; the representation is described here: http://www.thingiverse.com/thing:2289738
There may be some bugs in the optimization. If you run into p