C/C++ keyword conflicts: `operator` and `new`

[charlesneimog]

The C code uses identifiers operator and new, which conflict with C++ keywords. Would you consider renaming them to something like op and nnew?

I have a PR where I rename all these variables: https://github.com/charlesneimog/libonnx/tree/rename-cxx-conflicts. It touches almost all files in the codebase, and the variable names I chose might not be ideal.

In any case, thank you for the great work—this is an amazing repository to learn how these models work.