Generate VTables automatically

[laurooyen]

Generate the vtables in slang-sys with bindgen instead of writing them manually.
This should be possible once bindgen supports vtables for inherited classes.

See: rust-lang/rust-bindgen#2799

[laurooyen]

Since Slang welcomes contributions for language bindings, I'm currently focussing my efforts on implementing these upstream. Initial work will limit itself to the low level bindings i.e. slang-sys. This will already ensure we won't need to write VTables manually. Since low level struct and enum bindings will be generated too, we could ditch bindgen altogether in the future.

[valenotary]

@laurooyen could you link your upstream effort? I'm curious about your approach. Not someone particularly knowledgable on Rust but would love to learn more, and definitely am curious about Slang.