FIX #2845: Allow breakpoints on expressions with unlifted type

FIX #2845: Allow breakpoints on expressions with unlifted type

It turns out we can easily support breakpoints on expressions with
unlifted types, by translating

  case tick# of _ -> e

into

  let f = \s . case tick# of _ -> e
  in  f realWorld#

instead of just a plain let-binding.  This is the same trick that GHC
uses for abstracting join points of unlifted type.

In #2845, GHC has eta-expanded the tick expression, changing the
result type from IO a to (# State#, a #), which was the reason the
tick was suddenly being ignored.  By supporting ticks on unlifted
expressions we can make it work again, although some confusion might
arise because _result will no longer be available (it now has
unboxed-tuple type, so we can't bind it in the environment).  The
underlying problem here is that GHC does transformations like
eta-expanding the tick expressions, and there's nothing we can do to
prevent that.