Compress Ring.Link implementation slightly

The temporary variable names were for my 1:30am brain, but they don't
actually add any clarity by virtue of only being 1 character long.

However, giving them long names probably won't help either as there's
just a lot of mental parsing overhead that's required when visualizing
linked list operations anyway.

So let's just nuke them and do it all in-line as a pair of multi-valued
assignments, with slightly better comments.

The first assignment is equivalent to 'b.next' and 'y.next' in the old
code and covers linking self's tail node to other's head node, and
other's tail node to self's head node.

The second assignment is for 'a.prev' and 'x.prev' in the old code; this
links our head node backwards to other's tail node, and other's head
node back to our tail node.

The second requires the use of a multi-value assignment, as it's effectively
an exchange of values. In a multi-value assignment statement all
expressions on the right of the '=' are evaluated first with their
results stored on the stack, and then afterwards the actual assignments
to the things on the left takes place.

AceComm-3.0/ChatThrottleLib.lua

@@ -121,17 +121,10 @@ function Ring:Link(other)  -- Move and append all contents of another ring to th
 		self.pos = other.pos
 		other.pos = nil
 	elseif other.pos then
-		-- Link the other ring to the back of this one.
-		local a = self.pos
-		local b = self.pos.prev
-		local x = other.pos
-		local y = other.pos.prev
-
-		a.prev = y  -- My first points to their last...
-		y.next = a  -- ...and their last to my first.
-		b.next = x  -- My last points to their first...
-		x.prev = b  -- ...and their first to my last.
-
+		-- Our tail should point to their head, and their tail to our head.
+		self.pos.prev.next, other.pos.prev.next = other.pos, self.pos
+		-- Our head should point to their tail, and their head to our tail.
+		self.pos.prev, other.pos.prev = other.pos.prev, self.pos.prev
 		other.pos = nil
 	end
 end