[decompiler] Several small fixes (#775)

* fix assorted bugs

* stricter return types

* only look for functions in code files

test/decompiler/reference/kernel/gkernel_REF.gc

@@ -1508,11 +1508,11 @@
    (when parent
     (let ((child (-> parent 0 child)))
      (if (= child proc)
-      (return (the-as (pointer process-tree) #f))
+      (return (the-as object #f))
       )
      (while child
       (if (= (-> child 0 brother) proc)
-       (return child)
+       (return (the-as object child))
        )
       (set! child (-> child 0 brother))
       )