bug: overwriting trixls isn't properly supported.

js/pixl/README.md

  the `keys()` function on maps. i didn't find any convenient polyfills
  (`keys()` should return something iterator-like, e.g. returning raw
  values on `next()` and `StopIteration` when the end is reached.)
* bug: overwriting trixls isn't supported as `Map#set` compares by
  reference and not by value. maybe use chunks instead? (could be a
  performance boost as well.)