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

**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?**
* [x] Are you? :smiley: