Ensure push completes first.
Ensure a closed channel receives no new pushes
Pushing to a closed channel is not a second class Select case
Only complete Push if it's incomplete
Revamp Channel specs
Fail on second default regardless of completion
Ensure channels deny new shifts after being closed
Fixup channels
Add `#select` to `going/kernel`
Do not call completion block if an error occurs
Add final select specs
Register operation before completing it
Merge pull request #21 from jridgewell/cleanup

Cleanup