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/eval.hh

@@ -206,7 +206,7 @@ private:
     void addPrimOp(const string & name,
         unsigned int arity, PrimOpFun primOp);
 
-    inline Value * lookupVar(Env * env, const VarRef & var, bool noEval = false);
+    inline Value * lookupVar(Env * env, const VarRef & var, bool noEval);
     
     friend class ExprVar;
     friend class ExprAttrs;