Refactor grammars, introduce Labels, and Label Aspects

 * Expression grammars are now alwayse 'Labels -> Expr', simple grammars
   that are 'Type -> Expr' can simply extend SimpleExpressionGrammars

 * Labels now have Aspects. A label aspect describes a particular
   feature of the label (e.g. its size), as well as how this aspect
   applies to sub-productions.

   Aspects describe how sub-productions are generated/filtered/modified.
   See comments in Aspect for more information