Add support for encoding ArrayBuffers

[gacelita]

This allows users to upload files via Websockets using Fressian as transport format (jahoren/Chord).

(let [fr (js/FileReader.)
      file (-> some-js-event .-target .-files (aget 0))]
  (set! (.-onload fr) #(send-this-to-chord (-> fr .-result)))
  (.readAsArrayBuffer fr file))