Increase purity by introducing F[_] to Sink, Source and Cancelable

[busti]

This pull request aims to add polymorphic F[_] effect types to the onNext, onError and subscribe functions found in Sink and Souce. This is to make integrating libraries like zio_stream or fs2 easier and most importantly safer / purer.

The pull request is based on #122 since it makes it a lot easier to implement F[_] in many places.

This is still a work in progress, at the moment this PR mostly serves to keep people informed about the progress on this and to encourage discussion.

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This change was previously being tracked in #123 but unfortunately the branch got deleted, which resulted in closing the PR.
I wanted to rename the branch while the PR was open, which github should support when done from inside github, but that seems to have failed.