Datalog/DoC: add fast path for join_project for the case 'h(X) :- f(X), g(X).'

Signed-off-by: Nuno Lopes <a-nlopes@microsoft.com>

src/muz/rel/udoc_relation.h

@@ -82,6 +82,7 @@ namespace datalog {
         friend class udoc_relation;
         class join_fn;
         class join_project_fn;
+        class join_project_and_fn;
         class project_fn;
         class union_fn;
         class rename_fn;