Add withAsyncThrow? #161

I often need to run some important background task which I don't have to wait on and should be cancelled when I'm done.
However, it should interrupt me if it fails.

I've often needed it when writing tests and have been bitten by using withAsync bg (const fg) and having issues with the background task silently failing.

One possible pattern is to remember to always use link in these cases, but link turns synchronous exceptions into asynchronous ones which is a bit weird and not the most ergonomic pattern.

How about having something like the following? Would make link unnecessary for spawning threads you don't need to wait on.

withAsyncThrow :: IO a -> IO b -> IO b
withAsyncThrow left right =
  withAsync left $ \l -> 
  withAsync right $ \r -> do
  e <- waitEither l r
  case e of
    Left _ -> wait r
    Right v -> return v

Might need a better name (withConcurrent?) but others seem to have come across this need in the past too #128