Extensible languages in Clojure, a la Racket's #lang.

This is a simple library that monkey patches clojure.core/load
to be extensible to different backends.

`monkey-patch-extensible-load` does the actual monkey-patching and
must be called explicitly.

`lang-dispatch` is a map from keywords to alternative `load` functions
(of type [String -> nil]). The corresponding function will be used to
load a file according its :lang metadata entry in the `ns` form.

To add a new implementation, use
  (alter-var-root lang-dispatch assoc :new-impl my-load)

eg. A file with a `ns` form
      (ns fancy-ns-form
        {:lang :new-impl})
    will use `my-load` to load the file.