Add set_unfold_list_bind (+ test)

set_unfold_bind for sets already exists; this brings the list variant on par.

tests/sets.v

@@ -2,3 +2,8 @@ From stdpp Require Import sets.
 
 Lemma foo `{Set_ A C} (x : A) (X Y : C) : x ∈ X ∩ Y → x ∈ X.
 Proof. intros Hx. set_unfold in Hx. tauto. Qed.
+
+(** Test [set_unfold_list_bind]. *)
+Lemma elem_of_list_bind_again {A B} (x : B) (l : list A) f :
+  x ∈ l ≫= f ↔ ∃ y, x ∈ f y ∧ y ∈ l.
+Proof. set_solver. Qed.