Merge #466: Hot Fix: Use fuzzing not feature = "fuzzing"

0c15c01 Use fuzzing not feature = "fuzzing" (Tobin C. Harding)

Pull request description:

  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~, found while trying to [upgrade secp in bitcoin](rust-bitcoin/rust-bitcoin#1066).

  This got past CI because of the feature gate combination `#[cfg(all(test, feature = "unstable"))]`, we never run tests on CI with both DO_FEATURE_MATRIX and DO_BENCH.
  ```
  if [ "$DO_FEATURE_MATRIX" = true ]; then
  ...
      if [ "$DO_BENCH" = true ]; then  # proxy for us having a nightly compiler
          cargo test --all --all-features
          RUSTFLAGS='--cfg=fuzzing' RUSTDOCFLAGS='--cfg=fuzzing' cargo test --all --all-features
      fi
  fi
  ```

ACKs for top commit:
  apoelstra:
    ACK 0c15c01

Tree-SHA512: 08ada4eb20c3b7b128a225ed66cc621af097367f8ca19128b868d1b5de897f46d19f3a96a06ebd5dfaa288bc4477046f5d1214f0cdc33237b0ace079c539fc9e

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);