Don't suppress warnings for experimental Kotlin features

[Egorand]

Since Contour is using experimental features (inline classes), it should be either propagating the warnings through annotations or Gradle flags (see Experimental API markers), or introducing its own experimental annotation (see an example of this in KotlinPoet). An argument in favor of a library-specific annotation is that developers might be fine with using experimental features in their own code, but reluctant to use 3rd party libraries that use them, hence the library should require explicit opt-in.

[saket]

If apps are reluctant to use libraries that depend on inline classes then they'll have to avoid contour entirely because it uses them for describing layout specs. Should we instead call-it out in the readme?

[JakeWharton]

I think that would be wise in addition to correctly propagating either the built-in inline class opt-in annotation onto callers or defining your own.