Fix double precision conformance issues exposed by https://github.com/miloyip/nativejson-benchmark

[chadaustin]

See https://rawgit.com/miloyip/nativejson-benchmark/master/sample/conformance.html#1.%20Parse%20Validation

[aggsol]

Is this still on your agenda?

[chadaustin]

Hi @aggsol -- the only conformance issues are around the precision of very large and very small doubles, and when I looked into this last it looked like the only precision issues were at the least-significant digits. That said, I do think this is worth improving. I haven't prioritized it because many JSON encoders have precision issues for large and tiny doubles and I've yet to need it. If you'd like, it might be fun to write up a test case and see why precision is getting lost.