Add coverage to CI

.gitlab-ci.yml

 image: "archlinux:latest"
 
 before_script:
-  - pacman -Syu --needed --noconfirm rust gcc cargo-audit pkgconf
+  - pacman -Syu --needed --noconfirm rust gcc pkgconf cargo-audit cargo-tarpaulin
 
 stages:
   - lint
@@ -26,3 +26,11 @@ test:
   stage: test
   script:
     - cargo test --all --release
+
+coverage:
+  stage: test
+  script:
+    - cargo tarpaulin --out Xml
+  artifacts:
+    reports:
+      cobertura: cobertura.xml