- This patch implements `Task::Queue` mirroring `Thread::Queue` in CRuby.
- Producer/Consumer pattern is now possible with no polling
## No Top-level pollution
- No top-level `Queue` defined
- No `TaskError`. Instead, `Task::Error` happens when `Task::Queue#pop(true)` when empty
- No `ClosedQueueError`. `Task::Error` also happens when pushing to closed queue
## Future Work
- `Task::SizedQueue`
HASUMI Hitoshi