Derive Debug and eq. comparison traits for InvalidRustTarget enum

These traits are useful to get expressions like `RustTarget::stable(82,
0).unwrap()` to build, as `unwrap()` requires such a `Result` to
implement `Debug`. The equality comparison traits may also be useful in
cases where pattern matching the enum variants through e.g. `matches!`
or match expressions is deemed less stylish.

bindgen/features.rs

@@ -67,6 +67,7 @@ enum Version {
     Nightly,
 }
 
+#[derive(Debug, PartialEq, Eq, Hash)]
 pub enum InvalidRustTarget {
     TooEarly,
 }