Asyncronous serialization/deserialization

1. Async in deserialization is quite rough and dependent on the amount of custom data -- maybe we need to expose like `.handle_primive(x)` (or all internals + document them) or something to insert yields into strictly each Nth node?
2. Provide a built-in async option to serializer? Like `ldump.serializer.async = ldump.estimate_fps(60, love.timer.getTime)`?