Better handling for (pthread) cancellation.

[bakpakin]

Even if Janet doesn't explicitly allow cancellation of threads, other C functions may call pthread_cancel on threads. We should investigate making sure the interpreter integrates with pthead cancellation points such that there isn't any (or minimal) chance of corruption on thread cancellation.

[pyrmont]

I apologise for what is essentially a '+1' post but I'd quite like the ability to cancel Janet threads for a project that I'm working on.