ci: add clippy

Same as http-rs/tide#618

Refs: http-rs#199

.github/workflows/ci.yaml

@@ -48,7 +48,7 @@ jobs:
         args: --all --no-default-features --features '${{ matrix.backend }} middleware-logger encoding'
 
   check_fmt_and_docs:
-    name: Checking fmt and docs
+    name: Checking fmt, clippy, and docs
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@master
@@ -58,6 +58,9 @@ jobs:
         components: rustfmt, clippy
         override: true
 
+    - name: clippy
+      run: cargo clippy -- -D warnings
+
     - name: fmt
       run: cargo fmt --all -- --check