Make `cancel` uninterruptible...

[bitonic]

...create interruptibleCancel, update docs.

CC @snoyberg @kantp

[bitonic]

Note that this fixes #43

[simonmar]

@snoyberg you said here #41 (comment) that you wouldn't mind the waitCatch in cancel being interruptible, but what we ended up with (I only just realised) is that it is uninterruptible in the context of withAsync, and this ticket argues for it always to be uninterruptible.

Do we really want either of those?

[bitonic]

@simonmar me and @snoyberg had various discussions after that comment and he agreed that uninterruptible cancel by default is the way to go. I think he'll also be available later today so he can reply himself :)

[simonmar]

could you make fde3c6d a separate pull request please?

[snoyberg]

I was mistaken in that comment, as @bitonic convinced me. We definitely need to deliver the exception to the child thread, so making the cancel uninterruptible makes sense. There's still an argument for making the waitCatch interruptible.

[simonmar]

@snoyberg it was the waitCatch I was talking about (and what your earlier comment was about). Since uninterruptibleCancel now includes the waitCatch, the waitCatch in withAsync is now uninterruptible. Is that what we want?