This avoids constructing two unordered maps and numerous vectors for
each IL level at library load time for each plug-in that uses the C++
API. Additionally, the arrays allow for more efficient look-ups.
When compiling in C++ mode, the C++ standard library headers should be
included instead of their C counterparts.
Drive-by:
- Removed include of `json/json.h` from `settings.cpp`, where it was not
used
