Right now, we only have

data AsyncException
  = StackOverflow
  | HeapOverflow
  | ThreadKilled
  | ...

so it is not possible to add another async exception.  For instance,
the Timeout exception in System.Timeout should really be an async
exception.

This patch adds a superclass for all async exceptions:

data SomeAsyncException = forall e . Exception e => SomeAsyncException e
  deriving Typeable

and makes the existing AsyncException and Timeout children of
SomeAsyncException in the hierarchy.