optimizer misses flonum opportunities in `(apply + ...)` and `foldl`

The optimizer will use unsafe-fl+ for direct uses of + with flonums. Very nice. Even better, it even works variadically (which unsafe-fl+ ordinarily does not).

But when you change to (apply + ...) or foldl, you lose the optimization:

#lang typed/racket
(require math)

(+ 1.0 2.0) ; optimized
(+ 1.0 2.0 3.0) ; optimized
(apply + '(1.0 2.0 3.0)) ; not optimized
(foldl + 0.0 '(1.0 2.0 3.0)) ; not optimized
(foldl fl+ 0.0 '(1.0 2.0 3.0)) ; not optimized

The lower three lines could be better expanded to either

(unsafe-fl+ (unsafe-fl+ 1.0 2.0) 3.0)

or

(foldl unsafe-fl+ 0.0 '(1.0 2.0 3.0))

Though I think the apply form is the worthiest one to optimize, because it’s the more common idiom, and would subsume the foldl versions.