The input mode for Deserializer meant that the caller needed to manage the input stream and push data in an appropriate way; this is error-prone. Now Deserializer manages the input stream and the modes it must be read from. Serializer still works in a similar fashion as before, but now it deals with literal data more efficiently, writing it to the output stream directly (via splice_async()) rather than into the in-memory temporary buffer. Serializable's serialize() method is now async, meaning that all serialization can occur asynchronously, which is how we want it going forward.
