Also provide wpi::mutex and wpi::condition_variable as wrappers for these
on Linux (where they're available), and for std::mutex and
std::condition_variable on other platforms.
This is a modified version of https://github.com/nlohmann/json.
It's been moved into the wpi namespace as many of the changes are not
compatible. The amount of template code has been significantly reduced,
enabling many functions to be moved out-of-line, and for the result to
build on older compiler versions (in particular GCC 4.8).
