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.