Ported the core of the overlays management subsystem from /tg

- Added SSoverlays subsystem for compiling overlay lists and applying them to atoms in a controlled anti-lag subsystem.
- Added vars and procs to atom which should eventually replace all direct interaction with BYOND's /atom/overlays var outside the subsystem.
- Added OVERLAY_QUEUED flag to var/atom/flags bitfield.
- Added small framework for subsystem performance tracking. So far used only by SSoverlays
- Added admin debug command "Display overlay Log" to see performance stats mid-round.

code/_helpers/sorts/comparators.dm

@@ -36,3 +36,9 @@
 		. = b.head_position - a.head_position
 	if (. == 0) //Already in head/nothead spot, sort by name
 		. = sorttext(b.title, a.title)
+
+// Sorts entries in a performance stats list.
+/proc/cmp_generic_stat_item_time(list/A, list/B)
+	. = B[STAT_ENTRY_TIME] - A[STAT_ENTRY_TIME]
+	if (!.)
+		. = B[STAT_ENTRY_COUNT] - A[STAT_ENTRY_COUNT]