Implement lock-free, work-stealing, observable future dispatch

Latches make promises, relays form futures, and traces represent
execution contexts. Async implements a lock-free, work-stealing thread
pool with elidable monitoring methods.

Async uses efficient, immutable data structures internally, opening the
door for gratuitous introspection, and eliminating the ctl bottleneck
in Java's ForkJoinPool. And the reduction of ForkJoinPool's 4500 lines
of code to Async's mere 400 lines should convince you of the inevitable
ascent of Scala.