Benchmarking tool for React application performance

ReactAppPerf wraps core methods and logs info from them; there's no real
UI at this point
Explicit and Implicit Keys Need Separate Namespaces

There are certain cases where you can end up with a collision with an implicit
key (array index) if your explicit key prop is a number. They should use
different namespaces. Therefore I wrap explicit keys in curlies and implicit
array indices in brackets.

I added a simple test case, but another case came up on the mailing list. Where
undefined entries in an array actually results in an entry and therefore an
implicit key.