Scheme: Define default values for primitive shapes

[Kreyren]

On OpenSCAD i like that i can do cube(); for the interpreter to render a 1x1x1 cube which feels like it makes the finding of dimensions and relations much more efficient by reducing the need to write the whole cube([1,1,1], centered = true);

Where in libfive the use of cube or (cube) is illegal -> Proposing to default default value if no argument is parsed.

FWIW i tried to contibute this, but i have no idea how the C++ generator does it.