Support singletons in `solve_map_disjoint`.

tests/fin_maps.v

+From stdpp Require Import fin_maps.
+
+Section map_disjoint.
+  Context `{FinMap K M}.
+
+  Lemma solve_map_disjoint_singleton_1 {A} (m1 m2 : M A) i x :
+    m1 ##ₘ <[i:=x]> m2 → {[ i:= x ]} ∪ m2 ##ₘ m1 ∧ m2 ##ₘ ∅.
+  Proof. intros. solve_map_disjoint. Qed.
+
+  Lemma solve_map_disjoint_singleton_2 {A} (m1 m2 : M A) i x :
+    m2 !! i = None → m1 ##ₘ {[ i := x ]} ∪ m2 → m2 ##ₘ <[i:=x]> m1 ∧ m1 !! i = None.
+  Proof. intros. solve_map_disjoint. Qed.
+End map_disjoint.

theories/fin_maps.v

@@ -1932,6 +1932,8 @@ Hint Extern 2 (_ ##ₘ {[ _ := _ ]}) =>
   apply map_disjoint_singleton_r_2 : map_disjoint.
 Hint Extern 2 (_ ∪ _ ##ₘ _) => apply map_disjoint_union_l_2 : map_disjoint.
 Hint Extern 2 (_ ##ₘ _ ∪ _) => apply map_disjoint_union_r_2 : map_disjoint.
+Hint Extern 2 ({[_:= _]} ##ₘ _) => apply map_disjoint_singleton_l : map_disjoint.
+Hint Extern 2 (_ ##ₘ {[_:= _]}) => apply map_disjoint_singleton_r : map_disjoint.
 Hint Extern 2 (<[_:=_]>_ ##ₘ _) => apply map_disjoint_insert_l_2 : map_disjoint.
 Hint Extern 2 (_ ##ₘ <[_:=_]>_) => apply map_disjoint_insert_r_2 : map_disjoint.
 Hint Extern 2 (delete _ _ ##ₘ _) => apply map_disjoint_delete_l : map_disjoint.