First pass at IO::Notification.

Provides OS-backed notification of file changes. It's a fairly raw
stream; more sophisticated things based on it can likely evolve in
module space for the time being.

src/core/IO/Notification.pm

@@ -0,0 +1,27 @@
+my enum FileChangeEvent (:FileChanged(1), :FileRenamed(2));
+
+my class IO::Notification {
+    my class FileWatchCancellation is repr('AsyncTask') { }
+
+    class Change {
+        has $.path;
+        has $.event;
+    }
+
+    method watch_path($path as Str, :$scheduler = $*SCHEDULER) {
+        my $s = Supply.new;
+        nqp::watchfile(
+            $scheduler.queue,
+            -> \path, \rename, \err {
+                if err {
+                    $s.quit(err);
+                }
+                else {
+                    my $event = rename ?? FileRenamed !! FileChanged;
+                    $s.more(Change.new(:path(path // $path), :$event));
+                }
+            },
+            $path, FileWatchCancellation);
+        $s
+    }
+}

tools/build/Makefile-Moar.in

@@ -169,6 +169,7 @@ M_CORE_SOURCES = \
   src/core/IO/Socket.pm \
   src/core/IO/Socket/INET.pm \
   src/core/IO/Socket/Async.pm \
+  src/core/IO/Notification.pm \
   src/core/OS.pm \
   src/core/core_epilogue.pm \