[C API]: Update bindings to latest ONNX runtime release

[radu-matei]

The Rust bindings for the ONNX runtime are based on a fork of this project - https://github.com/nbigaouette/onnxruntime-rs, whose latest supported ONNX release is 1.6.

Given that we already use a fork of it, and because of some apparent performance issues with CUDA 10.2 (#9), we should update the bindings to ONNX 1.8, and eventually contribute the changes upstream.

[Mossaka]

Should we reach out to the maintainers of the onnxruntime-rs in case if they planed to support the latest ONNX runtime?

[radu-matei]

This doesn't seem to be planned, and an existing PR to update to 1.7 is still in progress - nbigaouette/onnxruntime-rs#78

Once we have a version of this running properly under 1.8, we can contribute the changes upstream.