EXPERIMENTAL: Returns a JavaScript Proxy ctor fn with the provided
key-fn. Invoking the returned fn on ClojureScript maps and vectors
will returned proxied values that can be used transparently as
JavaScript objects and arrays:

  (def proxy (builder))

  (def proxied-map (proxy {:foo 1 :bar 2}))
  (goog.object/get proxied-map "foo") ;; => 1

  (def proxied-vec (proxy [1 2 3 4]))
  (aget proxied-vec 1) ;; => 2

Access patterns from JavaScript on these proxied values will lazily,
recursively return further proxied values:

  (def nested-proxies (proxy [{:foo 1 :bar 2}]))
  (-> nested-proxies (aget 0) (goog.object/get "foo")) ;; => 1

Note key-fn is only used for proxied ClojureScript maps. This
function should map strings to the appropriate key
representation. If unspecified, key-fn defaults to keyword. All maps
proxied from the same ctor fn will share the same key-fn cache.