Add blessed.extended() helper to extend promises
Implement a guard helper for supposed promises-for-arrays
Test: Add sentinels helper
Test: Clarify some test(file)s

* Rename `cancellation-test` to "scope" it under `promise`
* Clarify naming of `subclass-test`
* Use `sentinels` helper
* Clarify method names when describing tests
Implement Collection subclass with sugar for promises-for-arrays
Add sugar to the Promise prototype
wip!