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Support for thread{WaitRead,WaitWrite,Delay}.  These should behave
identically to the 3.02 implementations.

We now have the virtual timer on during all program runs, which ticks
at 50Hz by default.  This is used to implement threadDelay, so you
won't get any better granularity than the tick frequency
unfortunately.  It remains to be seen whether using the virtual timer
will have a measurable impact on performance for non-threadDelaying
programs.

All operations in the I/O subsystem should now be non-blocking with
respect to other running Haskell threads.  It remains to be seen
whether this will have a measurable performance impact on
non-concurrent programs (probably not).

Some small pedantic changes to get function prototypes right.

Move 4.01 onto the main trunk.