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/pal/pal_timer_default.h

@@ -0,0 +1,32 @@
+
+#pragma once
+
+#include "pal_consts.h"
+#include "pal_ds.h"
+
+#include <chrono>
+
+namespace snmalloc
+{
+  template<typename PalTime>
+  class PalTimerDefaultImpl
+  {
+    inline static PalTimer timers{};
+
+  public:
+    static uint64_t time_in_ms()
+    {
+      auto time = PalTime::internal_time_in_ms();
+
+      // Process timers
+      timers.check(time);
+
+      return time;
+    }
+
+    static void register_timer(PalTimerObject* timer)
+    {
+      timers.register_timer(timer);
+    }
+  };
+}