Performance enhancements III

This commit includes:

* Rename "Executable" to "Process".
* Better documentation for Process.
* Modify existing Process implementations to better leverage
#background() via automated suspension checking to remove looping within
#runtime().
* Move all event-handling classes and modules to the new "events.rb"
source file to clean up "entity.rb".
* Better Event documentation.
* EventListener components may now be created using only a mask and
block to define its action logic.
* Minor readability improvements.