[project @ 1999-03-17 13:19:19 by simonm]

- Stack overflow now generates an (AsyncException StackOverflow)
  exception, which can be caught as normal.

- Add a stack overflow handler to the top-level mainIO handler, with
  the standard behaviour (i.e. call the stack overflow hook and then
  exit).

- Add a test for stack overflow catching.

- Fix a couple of bugs in async exception support.