Add CI (test)

.gitlab-ci.yml

+image: rust:alpine
+
+stages:
+    - lint
+    - test
+    - build
+    - fmt
+
+lint:
+    stage: lint
+    script:
+        - cargo clippy
+
+test:
+    stage: test
+    script:
+        - cargo test
+
+build:
+    stage: build
+    script:
+        - cargo build --release
+    artifacts:
+        paths:
+            - target
+fmt:
+    stage: fmt
+    script:
+        - cargo fmt
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