Functions API, easier single-thread use

• Added functions.php API for reactor use in procedural and functional code.
• ReactoryFactory::select() is now a static singleton method. Single-threaded code should never
  use multiple event loops. This change is made to ease Reactor procurement and minimize bugs
  from the existence of multiple Reactor instances in the same thread. It is NOT, however, an
  excuse to forego dependency injection. Do not abuse the global nature of the event loop. Lazy
  injection is fine, but laziness on your part as a programmer is not.

BC BREAKS:

• The ReactorFactory::__invoke() magic method has been removed. Any code relying on it must migrate
  references to ReactoryFactory::select()