libexpr: make ExprInheritFrom not be an ExprVar

this was only a convenient fiction when it was introduced, but it is no longer.

Change-Id: I72c50e7774c75408c1a40aee7da22059474ba01d

lix/libexpr/eval.cc

@@ -1259,6 +1259,14 @@ void ExprVar::eval(EvalState & state, Env & env, Value & v)
 }
 
 
+void ExprInheritFrom::eval(EvalState & state, Env & env, Value & v)
+{
+    Value * v2 = env.values[displ];
+    state.forceValue(*v2, pos);
+    v = *v2;
+}
+
+
 static std::string showAttrPath(EvalState & state, Env & env, const AttrPath & attrPath)
 {
     std::ostringstream out;

lix/libexpr/nixexpr.hh

@@ -138,20 +138,17 @@ struct ExprVar : Expr
  * Unlike normal variable references, the displacement is set during parsing, and always refers to
  * `ExprAttrs::inheritFromExprs` (by itself or in `ExprLet`), whose values are put into their own `Env`.
  */
-struct ExprInheritFrom : ExprVar
+struct ExprInheritFrom : Expr
 {
     ref<Expr> fromExpr;
+    Displacement displ;
 
     ExprInheritFrom(PosIdx pos, Displacement displ, ref<Expr> fromExpr)
-          : ExprVar(pos, {}), fromExpr(fromExpr)
+          : Expr(pos), fromExpr(fromExpr), displ(displ)
     {
-        this->level = 0;
-        this->displ = displ;
-        this->fromWith = nullptr;
     }
 
-    JSON toJSON(SymbolTable const & symbols) const override;
-    void bindVars(Evaluator & es, const std::shared_ptr<const StaticEnv> & env) override;
+    COMMON_METHODS
 };
 
 struct ExprSelect : Expr