Simplify inherited attribute handling

This reduces the difference between inherited and non-inherited
attribute handling to the choice of which env to use (in recs and lets)
by setting the AttrDef::e to a new ExprVar in the parser rather than
carrying a separate AttrDef::v VarRef member.

As an added bonus, this allows inherited attributes that inherit from a
with to delay forcing evaluation of the with's attributes.

Signed-off-by: Shea Levy <shea@shealevy.com>

src/libexpr/parser.y

@@ -413,7 +413,7 @@ binds
           if ($$->attrs.find(*i) != $$->attrs.end())
               dupAttr(*i, makeCurPos(@3, data), $$->attrs[*i].pos);
           Pos pos = makeCurPos(@3, data);
-          $$->attrs[*i] = ExprAttrs::AttrDef(*i, pos);
+          $$->attrs[*i] = ExprAttrs::AttrDef(new ExprVar(*i), pos, true);
       }
     }
   | binds INHERIT '(' expr ')' attrs ';'