Kotlinx collection immutable support

[beigirad]

We're replacing Kotlinx collection with normal collections in our project. But we can't use this types in json classes that Moshi requires.

There's some alternative JsonAdapters and Factories (like #449 and some other gist instances) but they're not as well as built-in adapters and hasn't full support like List and Map

[ZacSweers]

Is there a question here?

[beigirad]

@ZacSweers
No. I chose feature-request template. I have request to implement it as an artifact to extends Moshi features.

If you're agree with it, I can contribute for it.

[ZacSweers]

There's a similar request in moshix (moshi extensions), I think it'd make more sense there. The PR stalled, but perhaps you could offer to take it over - ZacSweers/MoshiX#478