use the same bindings print for ExprAttrs and ExprLet

this also has the effect of sorting let bindings lexicographically
rather than by symbol creation order as was previously done, giving a
better canonicalization in the process.

src/libexpr/nixexpr.hh

@@ -207,6 +207,8 @@ struct ExprAttrs : Expr
     ExprAttrs() : recursive(false) { };
     PosIdx getPos() const override { return pos; }
     COMMON_METHODS
+
+    void showBindings(const SymbolTable & symbols, std::ostream & str) const;
 };
 
 struct ExprList : Expr