FIX #1466 (partly), which was causing concprog001(ghci) to fail

FIX #1466 (partly), which was causing concprog001(ghci) to fail
An AP_STACK now ensures that there is at least AP_STACK_SPLIM words of
stack headroom available after unpacking the payload.  Continuations
that require more than AP_STACK_SPLIM words of stack must do their own
stack checks instead of aggregating their stack usage into the parent
frame.  I have made this change for the interpreter, but not for
compiled code yet - we should do this in the glorious rewrite of the
code generator.