Replace homebrewed make-js-map fn with built-in clj->js

CLJS/JS interop *used* to suck pretty hard, but things've improved somewhat over time – at least we don't need to hand-roll data structure conversion helpers anymore :)

There's also the `#js` reader tag, which could be used here as `#js {:id n :style s}` to produce the exact same result, but it looks like CLJSFiddle doesn't support that tag at the moment. Also note that `#js` isn't recursive whereas `clj->js` is – if I were to write `#js {:a {:b 1}}`, the result would be a JS object with a property `a` whose value is the CLJS map `{:b 1}`.