Add a line number error_info type for json parsing errors
Fix alternate char_type parsing.
Surrogate pairs are handled with char16_t (utf-16).
Make sure correct std::is* functions are called with templates.
Use std::codecvt_utf8* directly over std::wstring_convert to remove
extra exception handling & std::string usage
Add performance tests for utf16 & 32 parsing
Add a basic test for utf-16 & utf-32 parsing
Update benchmark results making sure all were compiled with identical
flags
Fix error_message() so that the line num doesn't screw with the line
position
Document json_reader support of alternate char-types & non-utf8 unicode