Draft: Use 'libclang' gem instead of system copy

Currently we require consumers of the Rust extension to have libclang installed on their hosts. This is needed for bindgen to create Rust bindings to libruby.

The rb-sys team has created a libclang gem that automatically installs that library. This lets us remove installing that package in CI and should make installation easier for consumers as well. This had a bug that prevented installation, but that is now fixed.

I have run into linking issues with libclang against systems with a static copy of libruby. We may need to hack around this by setting LIBCLANG_PATH if we detect a static library.

Also, specify the minimum supported Rust version as 1.65 in Cargo.toml.