Figure out if a thread is alive in `io-classes`

[MaximilianAlgehed]

Is your feature request related to a problem? Please describe.
With the removal of threadStatus support there is no way to figure out, to my best understanding, if a thread is alive or not.

Describe the solution you'd like
Since threadStatus has some issues in GHC, could we perhaps add something like isAlive :: ThreadId -> Bool which should have much clearer semantics?

Describe alternatives you've considered
Not being able to know if a part of your system is alive?

Additional context
This is usually possible in multi-threaded systems you'd like to model with io-sim...

Are you willing to implement it?

• Are you? 😃

[MaximilianAlgehed]

On a more serious note, this should be relatively straight-forward to implement without bumping into any of the really nasty bugs in GHC that caused this to be problematic before. For IO we should be able to do:

isAlive :: ThreadId -> IO Bool
isAlive tid = (`notElem` [ThreadFinished, ThreadDied]) <$> threadStatus tid

[coot]

The tricky part is to implement it in a way that IOSimPOR can discover races, e.g. if one thread is scheduled before the monitoring one, the monitoring one can receive a different result and thus land in a different execution branch.