A slew of SMP-related changes.

 - New locking scheme for thunks: we now check whether the thunk
   being entered is in our private allocation area, and if so
   we don't lock it.  Well, that's the upshot.  In practice it's
   a lot more fiddly than that.

 - I/O blocking is handled a bit more sanely now (but still not
   properly, methinks)

 - deadlock detection is back

 - remove old pre-SMP scheduler code

 - revamp the timing code.  We actually get reasonable-looking
   timing info for SMP programs now.

 - fix a bug in the garbage collector to do with IND_OLDGENs appearing
   on the mutable list of the old generation.

 - move BDescr() function from rts/BlockAlloc.h to includes/Block.h.

 - move struct generation and struct step into includes/StgStorage.h (sigh)

 - add UPD_IND_NOLOCK for updating with an indirection where locking
   the black hole is not required.