Merge kotlinx-coroutines-core and kotlinx-coroutines-jdk8

[qwwdfsad]

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We're gradually moving away from 1.6 support (#1589), it seems to be the time to merge supplementary jdk8 artifact into the core.

To keep Android API at a reasonable level, jdk8 will be existing in a separate source-set that will depend on core's main, but not the vice versa. Additionally, we'll keep releasing empty jdk8 artifact for compatibility reasons

[qwwdfsad]

As the change is rather technical, it seems like we can do it in a minor release as well to see how things are going prior to the same change in standard library