fixnum arithmetic is ignored

[sjamaan]

As reported by chickendan on IRC:

;; csc -O5 -strict-types -fixnum-arithmetic: 7.964s
;; csc -O5 -strict-types:                    8.105s
;; csc -O5 -strict-types (and fx functions): 0.246s
;;; c/nim -O3 baseline:                       0.211s

(define (fib n)
  (if (or (= n 0) (= n 1))
      n
      (+ (fib (- n 1)) (fib (- n 2)))))

(let loop ((n 0))
  (when (< n 35)
    (print "n=" n " => " (fib n))
    (loop (+ n 1))))

This program should use fixnum ops when declaring fixnum arithmetic. Also, when wrapping fib's body in an assume, it still compiles to C_s_a_i_plus calls rather than C_a_i_fixnum_plus or even just C_s_a_u_i_integer_plus.

I think we should take a look before releasing 5.1, the fact fixnum-arithmetic is ignored may not be a big issue, but it is rather worrying that the specializations aren't used.