Review the list of emit_x functions in Serializer.

[dpc]

While not tragic, every time this list changes, API breaks for serializer adapters (slog-serde).

Also, while at it, review the list of types implementing Serializable. Adding these is backward-compatible, but it's better to check now.

[dpc]

LGTM. Some notes:

• there is no sequences, as that would complicate everything; plus it's not a full serializer so no need for it, I believe
• there is no distinct emit_some as that would complicate everything; serialized value just knows if something was none or something, and will call the right method.