Use consistent type size_t for node_count() in C++ bindings

[volkm]

While trying to compile the C++ bindings, I had some compilation errors because node_count() different types.

This PR consistently uses uint64_t as default for node_count() because it seemed more widely used. One could of course also use size_t instead.

[nhusung]

Thank you! I’d rather opt for size_t everywhere. Somehow, clang++ and g++ did never complain about the type mismatches. Which compiler / OS did you use?

[volkm]

Thanks for integrating it.

BTW I used Clang 16.0.6 on the Apple M3 chip.