nqp-js/rakudo.js is now targeting ECMAScript 6

Scott McWhirter helped a ton with the transitions (as well as with some general cleanup).
Most of the modern browsers now support ECMAScript 6 so I feel it makes sense to target it.
When targeting old ones that don't we can use polyfills and compilers from ECMAScript 6 to 5.

After doing most of the obvious and promising nqp-js optimizations I'm focusing again on getting rakudo.js to work.
Before that I'm cleaning up the nqp-js code base to remove hacks that might shoot us in the back while working on rakudo.js/users/pawel_murias/2016/08/index.html