Add a basic cancellation object.

src/core/Cancellation.pm

@@ -0,0 +1,21 @@
+my class Cancellation {
+    has $.cancelled;
+    has $!lock;
+    has @!async_handles;
+
+    submethod BUILD(:@!async_handles) {
+        $!cancelled = False;
+        $!lock      = Lock.new;
+    }
+
+    method cancel() {
+        $!lock.protect({
+            unless $!cancelled {
+                for @!async_handles {
+                    nqp::cancel(nqp::decont($_));
+                }
+                $!cancelled = True;
+            }
+        })
+    }
+}

tools/build/Makefile-Moar.in

@@ -157,6 +157,7 @@ M_CORE_SOURCES = \
   src/core/Thread.pm \
   src/core/Lock.pm \
   src/core/Semaphore.pm \
+  src/core/Cancellation.pm \
   src/core/Scheduler.pm \
   src/core/ThreadPoolScheduler.pm \
   src/core/CurrentThreadScheduler.pm \