Typed Errors

Major update to deferred, adding typed errors as a generic parameter, using the same parameter names as the standard library's Result.

Another major change involves execution scheduling: execution now starts only once a Deferred's result has been explicitly requested in some way. The notify method is the primary way to accomplish this (along with onValue and onError), as well an explicit call to beginExecution().

Finally, the URLSessionTask wrapper has been moved to its own package, DeferredHTTP.