Allow ignoring default values

[nomisRev]

Hey @deusaquilus,
Thanks for this great library!

I am working with a lot of data, that contains a lot of optional values. Whilst pprint is already much better than toString, having all the redundant null is still quite annoying and produces a lot of noise.

KotlinX allows ignoring defaults using encodeDefaults = false, but not sure how helpful that is for the KMP implementation.