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
Test: Add util file

* Include method to test correct promise constructor behavior
Implement Collection subclass with sugar for promises-for-arrays
Implement promise racing sugar
Implement generic Promise sugar
wip! Implement sugar for delays and timeouts
wip! Implement sugar for managing concurrent operations
wip! Implement sugar for functional applications