Serialization Framework Tweaks

[TheBlueMatt]

Sadly a huge diff for only minor tweaks.

• First expose the serialization framework with some docs since it'll be needed for ChannelMonitor/ChannelManager serialization sooner or later anyway.
• Second, remove the newtype wrapper around std::io::Read/Write that wasn't really doing anything.
• Third, add size_hinting and fix little ineffeiency TODOs from the original serialization PR.
• Fourth, fix a bug introduced in the original serialization PR that caused messages to be encoded with their length instead of their message type.
• Finally, use the size_hinting in peer_handler and check that it never underestimates in fuzz testing.