ci: enable clippy

.github/workflows/ci.yaml

@@ -26,6 +26,7 @@ jobs:
       uses: actions-rs/toolchain@v1
       with:
         toolchain: ${{ matrix.rust }}
+        components: clippy
         override: true
 
     - name: check
@@ -40,6 +41,12 @@ jobs:
         command:  check
         args: --all --benches --bins --examples --tests --features "hyperium_http,unstable"
 
+    - name: clippy
+      uses: actions-rs/cargo@v1
+      with:
+        command: clippy
+        args: -- -D warnings
+
     - name: tests
       uses: actions-rs/cargo@v1
       with: