lazy *STACK-TOP-HINT*s

lazy *STACK-TOP-HINT*s

  Allow binding it to a symbol, which is resolved when entering the debugger:
  it denotes the name of the first uninteresting frame.

  This simplifies ERROR, CERROR, BREAK, %BREAK, and makes interrupts more
  efficient as we no longer need to find the interrupted frame when entering
  an interupt handler.

  It also makes (handler-bind ((error #'invoke-debugger)) ...) once again gain
  the benefits of the stack top hint, which we lost when we ceased the provide
  the hint around the call to SIGNAL for efficiency reasons. Best of both
  worlds, one hopes.