Add a timer to the PAL

This adds a way to periodically pool the PAL to see if any timers have
expired.  Timers can be used to periodically provide callbacks to the
rest of snmalloc.

src/ds/flaglock.h

@@ -2,6 +2,8 @@
 
 #include "bits.h"
 
+#include <atomic>
+
 namespace snmalloc
 {
   class FlagLock

src/ds/helpers.h

@@ -4,6 +4,7 @@
 #include "flaglock.h"
 
 #include <array>
+#include <atomic>
 #include <type_traits>
 
 namespace snmalloc