add c api bindings for lgraph

[antkiller996]

Here are c api bindings for lgraph.

The implementation is really intuitive.

• Every C++ class within lgraph_api namespace is ported to an opaque c struct struct lgraph_api_cxx_class_name_t.
• All public constructors are ported to c functions with signature like lgraph_api_cxx_class_name_t* lgraph_api_cxx_class_name_create(param1, param2, ...)
• All destructors are ported to c functions with signature like void lgraph_api_cxx_class_name_destroy(lgraph_api_cxx_class_name_t*)
• All public methods are ported to c functions with signature like return_type lgraph_api_cxx_class_name_method_name(param1, param2, ...). If there are multiple overloaded variants, there should be like return_type lgraph_api_cxx_class_name_method_name_{by, with}_param1(param1, param2, ...)
• C++ exceptions are ported to c string storing the error message returned from e.what()

[CLAassistant]

All committers have signed the CLA.

[wangtao9]

The first thing to remember is that the declaration of lgraph_api is in include directory, and its definition is in src/lgraph_api directory.

Logically c-bindings is part of lgraph_api, now the problem is that C API has been added into include, but its definition is not packed in ${TARGET_LGRAPH}. Please fix this in BuildLGraphApi.cmake.

[wangtao9]

In the implementation directory, it is recommended to rearrange this to src/lgraph_api/c.cpp.

[wangtao9]

I recommend merging the cmakelists file of build-c-bindings into BuildLGraphApi.cmake, since c-bindings should be part of lgraph_api

[antkiller996]

Now, c biindings are packed into lgraph_api target and src/c/c.cpp is moved to src/lgraph_api/c.cpp. @wangtao9