Change serialization to omit `none(T)` fields of sparse objects

[quantimnot]

I did this because I wanted a clean dump without all those nasty !!null ~'s that made my eyes weep. Let me know if this a no no, utter crap, or if should be behind a flag.

By the way, thank you very much for this excellent library. I use it all the time.

[quantimnot]

Please take another look. The force push was to amend the commit message.

[flyx]

Appreciated. Thanks!