Run clippy in the CI with warnings disallowed

As discussed previously it'd be nice to keep `ispc-rs` "free" of simple
"code smells" commonly pointed out by `clippy`.  This is easy to guard
by disallowing any clippy warnings from trickling into the codebase
through PRs via an extra CI check.

Downside is that Rust upgrades every 6 weeks and usually enables new or
improves existing lints, resulting in unrelated CI errors caused by
existing code being punted on unsuspecting contributors in newly opened
PRs.  But these are typically easy to resolve in a separate PR, or to be
caught early when (also) running the CI job on a schedule.

.github/workflows/main.yml

@@ -14,6 +14,7 @@ jobs:
             - run: curl -L https://github.com/ispc/ispc/releases/download/v${{ env.ISPC_VERSION }}/ispc-v${{ env.ISPC_VERSION }}-linux.tar.gz | tar xzv ispc-v${{ env.ISPC_VERSION }}-linux/bin/ispc
             - run: realpath "ispc-v${{ env.ISPC_VERSION }}-linux/bin/" >> $GITHUB_PATH
             - run: cargo build --all --all-targets --features ispc
+            - run: cargo clippy --all --all-targets --features ispc -- -D warnings
             - run: cargo test --all
             - run: cargo doc --all --no-deps --document-private-items --all-features
               env:
@@ -29,6 +30,7 @@ jobs:
                   toolchain: stable
             - run: brew install ispc
             - run: cargo build --all --all-targets --features ispc
+            - run: cargo clippy --all --all-targets --features ispc -- -D warnings
             - run: cargo test --all
     build_windows:
         runs-on: windows-latest
@@ -42,4 +44,5 @@ jobs:
                   unzip ispc-v${{ env.ISPC_VERSION }}-windows.zip ispc-v${{ env.ISPC_VERSION }}-windows/bin/ispc.exe
                   Resolve-Path "ispc-v${{ env.ISPC_VERSION }}-windows/bin" | Add-Content -Path $env:GITHUB_PATH
             - run: cargo build --all --all-targets --features ispc
+            - run: cargo clippy --all --all-targets --features ispc -- -D warnings
             - run: cargo test --all