Create intermediate sourceset to share between Apple and non-Apple targets

[ankushg]

In this PR:

• create nativeCoroutines as a sourceset between common and apple
• moved the contents of apple* into nativeCoroutines*
• created a cross-platform expect fun for freeze()
• created PlatformError as an expect class
  • exposed as NSError on Apple and Throwable on JS
  • create an internal kotlinCause extension that we that we can use instead of digging into userInfo so the tests work on non-Apple platforms

This sourceset can be used to house the Apple-agnostic code, and can eventually be used to share code across Apple and JS 😄

Notes:

• maybe it makes sense to keep the test cases in appleTest for now until we can successfully migrate to kotlinx-coroutine-test?

[rickclephas]

Looks good, let's move kotlinCause to nativeCoroutinesTest and merge this 😄