util.set: Add a serialization preparation metamethod

Enables util.serialization to turn Sets into a representation that can be
deserialized with an environment trick, i.e. `set{"a","b"}`. Also useful
for debug purposes.

util/set.lua

@@ -189,6 +189,15 @@ function set_mt.__tostring(set)
 	return t_concat(s, ", ");
 end
 
+function set_mt.__freeze(set)
+	local s = {};
+	for item in pairs(set._items) do
+		s[#s + 1] = item;
+	end
+	return s;
+end
+
+
 return {
 	new = new;
 	is_set = is_set;