engine: Rewrite the core algorithm

The engine core had some unfortunate bugs that were the result of some
early design errors when I wasn't as familiar with channels. I've
finally rewritten most of the bad parts, and I think it's much more
logical and stable now.

This also simplifies the resource API, since more of the work is done
completely in the engine, and hidden from view.

Lastly, this adds a few new metaparameters and associated code.

There are still some open problems left to solve, but hopefully this
brings us one step closer.

engine/error.go

@@ -24,9 +24,6 @@ type Error string
 func (e Error) Error() string { return string(e) }
 
 const (
-	// ErrWatchExit represents an exit from the Watch loop via chan closure.
-	ErrWatchExit = Error("watch exit")
-
-	// ErrSignalExit represents an exit from the Watch loop via exit signal.
-	ErrSignalExit = Error("signal exit")
+	// ErrClosed means we couldn't complete a task because we had closed.
+	ErrClosed = Error("closed")
 )