cancel blocking operations

[arjunguha]

Conversation with SamL:

if you want sleep() to be cancellable
it needs to cooperate
e.g., the person who writes sleep needs to have a handler that gets called by pause
so there needs to be an API change
i would say that pauseImmediate changes to have this type
ugh no its more complicated
you want to stop sleep, not pause it
stopify right now doesn't make a distinction
let's pretend it did (so first make that change), then
change the type to pauseImmediate(callback: (registerCancel : (handler: () => void) => void) => void): void