Trying out new scheme built-in function definitions. Allows built-ins to have default args and whatnot, and to be specified more easily than before.

Bam, re-did argument binding. More robust, and allows default arg values to refer to previoius args.

Forcing required params to precede optional params in function and mixin definitions, to match recent Sass changes.

Conflicts:
	functions.cpp

Making the color name to value conversion case-insensitive. Also handling the edge case of a function having the same name as a color.

Import tweaks

Fix bug in loop in Document::emit_css when size_t is unsigned.