guile, scheme: fix fn? when metadata contains "ismacro"

Let `with-meta f m` create a function even if f is a macro, instead of
setting an "ismacro" metadata that is never used again (and breaks
self-hosting).

Also move the test for fn? from Optional to Deferrable, the function
is used for self-hosting.

guile/core.scm

@@ -150,7 +150,7 @@
    ((callable? c)
     (let ((cc (make-callable ht
                              (callable-unbox c)
-                             (and (hash-table? ht) (hash-ref ht "ismacro"))
+                             #f
                              (callable-closure c))))
       cc))
    (else

scheme/lib/core.sld

@@ -275,7 +275,7 @@
                      ((func? x)
                       (let ((func (make-func (func-ast x) (func-params x)
                                              (func-env x) (func-fn x))))
-                        (func-macro?-set! func (func-macro? x))
+                        (func-macro?-set! #f)
                         (func-meta-set! func meta)
                         func))
                      (else

tests/stepA_mal.mal

@@ -64,6 +64,19 @@
 (meta +)
 ;=>nil
 
+;; Testing fn? function
+(fn? +)
+;=>true
+(fn? (fn* () 0))
+;=>true
+(fn? cond)
+;=>false
+(fn? "+")
+;=>false
+(fn? :+)
+;=>false
+(fn? ^{"ismacro" true} (fn* () 0))
+;=>true
 
 ;;
 ;; Make sure closures and metadata co-exist
@@ -139,18 +152,6 @@
 
 (def! add1 (fn* (x) (+ x 1)))
 
-;; Testing fn? function
-(fn? +)
-;=>true
-(fn? add1)
-;=>true
-(fn? cond)
-;=>false
-(fn? "+")
-;=>false
-(fn? :+)
-;=>false
-
 ;; Testing macro? function
 (macro? cond)
 ;=>true