Make solve_ndisj more powerful.

It now first turns hypotheses `X ∪ Y ⊆ Z` into `X ⊆ Z` and `Y ⊆ Z`.

opam.pins

-coq-stdpp https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp 613168bf8af3af0fe31873efd89038a89800792e
+coq-stdpp https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp 3103b7bf52d0275f2938d9af44ab2d0db89a6251

theories/base_logic/lib/namespaces.v

@@ -95,4 +95,8 @@ Hint Extern 1 (_ ⊆ _) => apply nclose_subseteq' : ndisj.
 Hint Resolve namespace_subseteq_difference_l | 100 : ndisj.
 Hint Resolve ndisj_difference_l : ndisj.
 
-Ltac solve_ndisj := solve [eauto with ndisj].
+Ltac solve_ndisj :=
+  repeat match goal with
+  | H : _ ∪ _ ⊆ _ |- _ => apply union_subseteq in H as [??]
+  end;
+  solve [eauto with ndisj].