Use fuzzing not feature = "fuzzing"

Currently the following command fails

`RUSTFLAGS='--cfg=fuzzing' RUSTDOCFLAGS='--cfg=fuzzing' cargo test --all --all-features`

This is because `fuzzing` is not a feature, we should be using `fuzzing`
directly not `feature = "fuzzing"`.

I have no idea how this got past CI.

src/key.rs

@@ -101,7 +101,7 @@ pub const ONE_KEY: SecretKey = SecretKey([0, 0, 0, 0, 0, 0, 0, 0,
 /// [`bincode`]: https://docs.rs/bincode
 /// [`cbor`]: https://docs.rs/cbor
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
-#[cfg_attr(feature = "fuzzing", derive(PartialOrd, Ord))]
+#[cfg_attr(fuzzing, derive(PartialOrd, Ord))]
 #[repr(transparent)]
 pub struct PublicKey(ffi::PublicKey);