Updated Selector::walk(Selector, iterator) to split the selector up into parts.

For example:

selector1: "a,b"
selector2: "b"

selector2.walk(selector1, selector2.begin());

The above will return selector2.end() rather than fail.
Adding LessAtRule.

Processes the value of the atrule. For example:

@atrule 1 + 1;

or

@atrule {
 "abc ${string} "
}