ir: Add @interface user attributes.
parser: Parse @interface user attributes.
semantic: Implement gather for UserAttributes.
ir: Add TraitsExp node.
parser: Parse TraitsExps.
rt: Add empty Attribute class to object.
ir: Add retrieveAttribute function.
ir: Add qualified name utility constructor.
semantic: Stub out getTraitsExpType.
semantic: Implement getTraitsExpType.
semantic: Wire UserAttribute into util.
ir: Add layoutStruct to UserAttribute.
semantic: Fill out layout struct on user attribute.
llvm: Simple stub wiring of UAs to llvm/type.
semantic: Mangle UserAttributes.
semantic: type getAttribute as a pointer to a UserAttribute.
semantic: Documentation.
semantic: Add qualified name lookup.

Treats all not last identifiers as scope, and the last as not.
ir: Add userAttribute field to Attribute.
semantic: Add fillInAttribute function.