This patch makes throwTo work with -threaded, and also refactors large
parts of the concurrency support in the RTS to clean things up.  We
have some new files:

  RaiseAsync.{c,h}	asynchronous exception support
  Threads.{c,h}         general threading-related utils

Some of the contents of these new files used to be in Schedule.c,
which is smaller and cleaner as a result of the split.

Asynchronous exception support in the presence of multiple running
Haskell threads is rather tricky.  In fact, to my annoyance there are
still one or two bugs to track down, but the majority of the tests run
now.