[decompiler] Several small fixes (#775)

* fix assorted bugs

* stricter return types

* only look for functions in code files

test/decompiler/reference/engine/math/vector-h_REF.gc

@@ -62,7 +62,7 @@
 ;; definition for method 9 of type bit-array
 (defmethod get-bit bit-array ((obj bit-array) (arg0 int))
   (let ((v1-2 (-> obj bytes (/ arg0 8))))
-   (nonzero? (logand v1-2 (ash 1 (logand arg0 7))))
+   (logtest? v1-2 (ash 1 (logand arg0 7)))
    )
   )