js-on-sx: TypeError-on-not-callable uses type-of, not (str fn-val)

Formatting wrapper dicts with (str fn-val) recursively walks the
proto chain through SX inspect — for String/Number wrappers whose
prototype contains lambdas this hangs. Switched the message to
(type-of fn-val), e.g. "dict is not a function". Less specific
but always terminates.
built-ins/String: 73/99 → 75/99 (canonical). conformance.sh:
148/148.

lib/js/runtime.sx

@@ -430,7 +430,7 @@
         (raise
           (js-new-call
             TypeError
-            (list (str (str fn-val) " is not a function"))))
+            (list (str (type-of fn-val) " is not a function"))))
         (cond
           ((= (len args) 0) (callable))
           ((= (len args) 1) (callable (nth args 0)))