unboxing in safe mode doesn't quite work

[felix winkelmann]

for example:

{{
(define (foo x y)

(fp+ (fp* x 23) y))

(print (foo 2 3))
}}

fails because C_a_i_flonum_plus gets passed an unboxed value.

[felix winkelmann]

Unboxing has now be enabled exclusively for unsafe mode. The safeness test (and in particular a failing one) would make the unboxing pass behave incorrectly.