The class is currently just a wrapper for an std::ostream and collects
all functions related to serialization. The next step should be
recycling of variables to avoid repetitive initialization for each
recursive dump call.

This reverts commit 909b439b.
For some strange reason, the test suite crashes when compiled
with GCC.

numtostr now directly writes to a stream. Return value of snprintf is
reused to avoid finding end of string. Cachegrind suggests a 1%
performance increase.

All ‘<<‘ calls have been replaced by write()/put() calls. The
indentation strings needs not to be resized. Cachegrind measures 1%
performance improvement.

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