Improve perf of io/file-attributes using CLJS Bean

[mfikes]

The CLJS Bean library provides a faster alternative to js->clj, and this can be used in io/file-attributes for a substantial speedup:

Before:

cljs.user=> (require '[planck.io :as io] '[planck.core :refer [file-seq]])
nil
cljs.user=> (let [files (doall (file-seq "/Users/mfikes/Documents"))]
       #_=>  (time (transduce (comp (map io/file-attributes) (map :reference-count)) + files)))
"Elapsed time: 31.728287 msecs"
1432

After:

cljs.user=> (require '[planck.io :as io] '[planck.core :refer [file-seq]])
nil
cljs.user=> (let [files (doall (file-seq "/Users/mfikes/Documents"))]
       #_=>  (time (transduce (comp (map io/file-attributes) (map :reference-count)) + files)))
"Elapsed time: 13.202166 msecs"
1432

[pyrmont]

Wow, that's pretty dramatic. Looks like a spiffy little library :)