Add --all-features when running clippy in ci

Currently we do not lint all features, this is not a problem since the
only feature guarded code is on the "std" feature which is on by
default. However, to future proof the crate, run clippy in CI with
`--all-features`.

Author: tcharding

.github/workflows/rust.yml

@@ -103,7 +103,7 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: clippy
-          args: --all-targets -- -D warnings
+          args: --all-targets --all-features -- -D warnings
 
   Fmt:
     name: Fmt