Changes to [de]serialization APIs

[copybara-service[bot]]

Changes to [de]serialization APIs

I was less radical than my TODO comment in serialize.lisp suggested:

• Changed the order of args for serialize-to-stream to match those of deserialize-from-stream.
• Made stream arg required for deserialize-from-stream. (Nobody is doing this from standard input.)
• Renamed serialize (an internal function that was (accidentally?) exported) to serialize-message.
• Removed deserialize, per the comment just above it.