event loop and event-driven async socket base class

 Danga::Socket is an abstract base class for objects backed by a socket which
 provides the basic framework for event-driven asynchronous IO, designed to be
 fast. Danga::Socket is both a base class for objects, and an event loop.
 .
 Callers subclass Danga::Socket. Danga::Socket's constructor registers
 itself with the Danga::Socket event loop, and invokes callbacks on the
 object for readability, writability, errors, and other conditions.
 .
 Because Danga::Socket uses the "fields" module, your subclasses must
 too.