Performance benchmarks for Foundation

As part of a PR for Foundation which aimed at improving the performance of
NSJSONSerialization (#718), I wrote a simple JSON serialization benchmark.

While I was developing the changes, I ran this benchmark by adding it to
the existing test suite in TestFoundation, but this isn't the right place
to put it. Is there already an appropriate place (perhaps under
apple/swift/benchmark), or should Foundation have its own benchmark suite?

- Dave