engine: resources: Add new value resource

This is a strange resource which is probably most useful for passing
values between scopes. It supports a variant resource field, and should
only be used as a last resort and if you know exactly what you're doing.

examples/lang/value-resource.mcl

@@ -0,0 +1,15 @@
+value "hello1" {
+	#value => 42,	# can be any type
+	value => "wow",	# can be any type
+}
+value "hello2" {
+	value => "whatever",	# TODO: remove the temporary placeholder here
+	#value => "",	# XXX: remove any placeholder to see the bug when absent
+}
+
+test "test" {
+	#anotherstr => "", # get it from send/recv
+}
+
+Value["hello1"].value -> Value["hello2"].value
+Value["hello2"].value -> Test["test"].anotherstr