Add `Serde::Serialize` for generated message types

[PolyMeilex]

Since we already depend on serde anyway, adding serialize allows for easy dumping of messages as JSON for quick debugging, which should be nicer than staring at raw tag integers.

While at it, I made FieldTag::from_tag_num public, which is also quite useful for looking up tag IDs when debugging without looking it up in XML.

[ldanko]

@PolyMeilex Merged. But it will have impact for build time which was quite long already.

Consider making this optional by adding serde feature to Cargo.toml