Implement null? predicate

scm/inc/compiler.scm

        (emit-expr (primcall-operand1 x))
        (emit "cmpl $0,  %eax") ; x == 0
        (emit-compare))
       ((null?)
        (emit-expr (primcall-operand1 x))
        (emit "cmpl $~a, %eax" #b00101111)
        (emit-compare))
       ((integer?)
        (emit-expr (primcall-operand1 x))
        (emit "andl $~a, %eax" #b11)
  (emit-expr x)
  (emit "ret"))
(compile-program '(boolean? #f))
(compile-program '(null? ()))