[Efficiency] Use ES6 Map()/Set() in lieu of Object where possible

[gorhill]

Benchmarks:

• Write ops: http://jsperf.com/smalles6set-vs-smallobject-write.
• Read ops: http://jsperf.com/smalles6set-vs-smallobject-read.

For instances in the code where no iteration through the set is needed, we can use a shim for versions of Chromium older than 38 -- Firefox supports ES6 Set() since FF 13.

Because for...of is supported only for Chromium 38+, for instances in the code where iterations through the set is needed, this will have to wait for longer term. Possibly a v2.0 of uBO will be incompatible with Chromium version 37 and older.

[gorhill]

Need to survey where else conversion to Set/Map is possible.

[gorhill]

https://jsperf.com/ is down, so there is a custom benchmark now: https://gorhill.github.io/obj-vs-set-vs-map/.

[kurtextrem]

@gorhill Thank you very much for doing that. First of all it is great to have an efficient blocker and at the same time an open development. The benchmark for Set/Map was interesting, I'm gonna use it as well.

[lewisje]

Also, in case anyone ports this to Safari: 9+ supports Map and Set very well, 7.1 and 8 support them fairly well, and older versions don't support them natively.