Resources
=========
* https://github.com/promises-aplus/promises-tests


Skipped
=======
* 2.3.3: Otherwise, if x is an object or function.
  This spec is a NOOP for Swift:
  - We have decided not to interact with other Promises A+ implementations
  - functions cannot have properties
* 2.3.3.4: If then is not a function, fulfill promise with x.
  - See: The 2.3.4 suite.