Allow using trait objects for CompileReflectShader

[chyyran]

This allows building on stable Rust, but not without some caveats.

• C headers will not be regenerated when building with the stable feature.
• There is a minor performance hit in initial shader compilation when building against stable Rust. This is due to boxed trait objects being used instead of impl Trait.
• A higher MSRV is required when building against stable Rust.