C wrapper for the C++ impl of the OpenTracing API

 OpenTracing C Wrapper library was created due to the need to use distributed
 tracing within C programs.
 .
 The OpenTracing project (https://opentracing.io/) has published a
 C-language implementation on https://github.com/opentracing/opentracing-c
 but that implementation is incomplete, with the last update on Jun
 27th 2018
 .
 This library takes inspiration from the header files in that project but
 implements the proposed functionality fully by wrapping the functional
 C++ library.