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/game/atoms.dm

@@ -22,6 +22,10 @@
 	// replaced by OPENCONTAINER flags and atom/proc/is_open_container()
 	///Chemistry.
 
+	// Overlays
+	var/list/our_overlays	//our local copy of (non-priority) overlays without byond magic. Use procs in SSoverlays to manipulate
+	var/list/priority_overlays	//overlays that should remain on top and not normally removed when using cut_overlay functions, like c4.
+
 	//Detective Work, used for the duplicate data points kept in the scanners
 	var/list/original_atom
 	// Track if we are already had initialize() called to prevent double-initialization.