Move .set .include & .label code into Directives

These directives should be handled by the Directives module, not the
Assembler class.  The .label directive is actually written by the end
user as (label-name), but the Line class works with the LabelDirective
class to transform the label text into the 'directive' form.
Ex:  `(my-label)` becomes .label `(my-label)`