fix: feature flag Datagram::into_data (#1695)

* feat(.github): run cargo clippy on each crate individually

> Use cargo-hack to run clippy on each crate individually with its
> respective default features only. Can reveal warnings otherwise
> hidden given that a plain cargo clippy combines all features of the
> workspace. See e.g. #1695.

* fix: feature flag Datagram::into_data

`Datagram::into_data` is used in `neqo_common::udp` only. `neqo_common::udp` is
behind the `udp` feature. The feature is not part of the crate's default
features. Thus a plain `cargo check` rightly complains with:

```
warning: method `into_data` is never used
  --> neqo-common/src/datagram.rs:58:19
   |
20 | impl Datagram {
   | ------------- method in this implementation
...
58 |     pub(crate) fn into_data(self) -> Vec<u8> {
   |                   ^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default
```

This commit hides `into_data` behind the `udp` feature as well.

* Install cargo-hack as part of 'Install Rust tools'

.github/actions/rust/action.yml

@@ -32,7 +32,7 @@ runs:
 
     - name: Install Rust tools
       shell: bash
-      run: cargo +${{ inputs.version }} binstall --no-confirm cargo-llvm-cov cargo-nextest flamegraph
+      run: cargo +${{ inputs.version }} binstall --no-confirm cargo-llvm-cov cargo-nextest flamegraph cargo-hack
 
     - name: Use Rust cache
       uses: Swatinem/rust-cache@v2

.github/workflows/check.yml

@@ -142,7 +142,12 @@ jobs:
         if: success() || failure()
 
       - name: Clippy
-        run: cargo +${{ matrix.rust-toolchain }} clippy --all-targets -- -D warnings || ${{ matrix.rust-toolchain == 'nightly' }}
+        run: |
+          # Use cargo-hack to run clippy on each crate individually with its
+          # respective default features only. Can reveal warnings otherwise
+          # hidden given that a plain cargo clippy combines all features of the
+          # workspace. See e.g. https://github.com/mozilla/neqo/pull/1695.
+          cargo +${{ matrix.rust-toolchain }} hack clippy --all-targets -- -D warnings || ${{ matrix.rust-toolchain == 'nightly' }}
         if: success() || failure()
 
       - name: Check rustdoc links

neqo-common/src/datagram.rs

@@ -54,6 +54,7 @@ impl Datagram {
         self.ttl
     }
 
+    #[cfg(feature = "udp")]
     #[must_use]
     pub(crate) fn into_data(self) -> Vec<u8> {
         self.d