v0.2.0-M2

For fs2 1.0.2, cats-effect 1.0.1 and cats-collections 0.7.
This release features significant changes, and it's probably how 0.2.0 final is going to look like.
Changes include:

• Limiter and Worker are now merged, Resource is used to manage the limiter lifetime.
• More flexible control over the rate.
• await and backpressure functionality is no longer baked in the core abstraction, and it's implemented as combinators instead.