Indentation string is recycled to avoid allocations. Comma-separation
in objects does not need an if any more. Cachegrind measures 1%
performance improvement.

Added separate code paths for normal output and pritty-printed output.
This allowed to remove most of the ifs along the way. Benchmarks and
cachegrind suggest a 10% performance improvement.

🔀 merge branch 'cotire' of https://github.com/tusharpm/json into tusharpm-cotire

- get CMake for XCode 8.1 image

- Add prefix header
  - Include catch.hpp
  - Include json.hpp
    - Replace private with public for all json_unit files
- Move `unit.cpp` to an object library
- cotire issue: strip whitespace from CMAKE_INCLUDE_SYSTEM_FLAG_CXX