fix ffi retain of callbacks

The FFI's weak table of callback procedures (to map Racket procedures
to FFI callback objects) suffered from the classic key-in-value
problem.

Closes PR 12228, probably

Merge to 5.2
(cherry picked from commit 8bd81f4806)

collects/ffi/unsafe.rkt

@@ -446,7 +446,7 @@
       (lambda (x)
         (and x
              (let ([cb (ffi-callback (wrap x) itypes otype abi atomic? async-apply)])
-               (cond [(eq? keep #t) (hash-set! held-callbacks x cb)]
+               (cond [(eq? keep #t) (hash-set! held-callbacks x (make-ephemeron x cb))]
                      [(box? keep)
                       (let ([x (unbox keep)])
                         (set-box! keep

collects/ffi/unsafe/try-atomic.rkt

@@ -67,7 +67,7 @@
 (define (can-try-atomic?) (and (freezer-box) (not (in-try-atomic?))))
 
 ;; prevent GC of handler while it's installed:
-(define saved-ptrs (make-hash))
+(define saved-ptrs (make-hasheq))
 
 (define (try-atomic thunk default 
                     #:should-give-up? [should-give-up?