Use performant Object.create replacement

[parshap]

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Object.create is much slower than using the new operator in some versions of V8 (confirmed in Chrome 33 using V8 3.23, see this jsperf). This changes calls to Object.create() to a semantically-equivalent createObject() helper function that creates objects using a constructor function and the new operator.

This change also adds a basic benchmark script that can be run in node using npm run bench or served to the browser using npm run bench-server.

My benchmarks before the change:

create: 794.785ms
overwrite: 791.635ms
set: 800.428ms

And after the change:

create: 19.083ms
overwrite: 17.687ms
set: 15.335ms