Add API for custom (de)serializers

builtin/common/metatable.lua

@@ -1,14 +1,28 @@
 -- Registered metatables, used by the C++ packer
+local serializable_metatables = {}
 local known_metatables = {}
-function core.register_portable_metatable(name, mt)
+
+local function dummy_serializer(x)
+	return x
+end
+
+function core.register_portable_metatable(name, mt, serializer, deserializer)
+	serializer = serializer or dummy_serializer
+	deserializer = deserializer or function(x) return setmetatable(x, mt) end
 	assert(type(name) == "string", ("attempt to use %s value as metatable name"):format(type(name)))
 	assert(type(mt) == "table", ("attempt to register a %s value as metatable"):format(type(mt)))
+	assert(type(serializer), ("attempt to use a %s value as serializer"):format(type(serializer)))
+	assert(type(deserializer), ("attempt to use a %s value as serialier"):format(type(deserializer)))
 	assert(known_metatables[name] == nil or known_metatables[name] == mt,
 			("attempt to override metatable %s"):format(name))
 	known_metatables[name] = mt
 	known_metatables[mt] = name
+	serializable_metatables[mt] = serializer
+	serializable_metatables[name] = deserializer
 end
+
 core.known_metatables = known_metatables
+core.serializable_metatables = serializable_metatables
 
 function core.register_async_metatable(...)
 	core.log("deprecated", "core.register_async_metatable is deprecated. " ..