CI: Add a clippy job

Uses stable clippy for the time being

Otherwise we'll get warnings about lint names being deprecated without
being able to fix it while still being able to compile on stable.

This will solve itself once the new lint names move to stable and we can
switch to them.

.gitlab-ci.yml

@@ -1,6 +1,7 @@
 stages:
   - "lint"
   - "test"
+  - "extras"
 
 .tarball_setup: &setup
   variables:
@@ -74,3 +75,12 @@ rustfmt:
     - rustup component add rustfmt
     - cargo fmt --version
     - cargo fmt -- --color=always --check
+
+clippy:
+  image: "rust:slim"
+  stage: 'extras'
+  <<: *setup
+  script:
+    - rustup component add clippy-preview
+    - cargo clippy --version
+    - cargo clippy --color=always --all --all-features