collapseos/blk/387

( handle backspace: go back one char in IN>, if possible, then
  emit BS + SPC + BS )
: _bs
    ( already at IN( ? )
    IN> @ IN( = IF EXIT THEN
    IN> @ 1- IN> !
    BS SPC BS
;
( del is same as backspace )
: BS? DUP 0x7f = SWAP 0x8 = OR ;
SYSVARS 0x55 + :** KEY
( cont.: read one char into input buffer and returns whether we
  should continue, that is, whether CR was not met. )