Add basic CI integration.

For now this is only running our catch tests, but we plan on adding more (ThreadSanitizer, basic performance metrics) later on.

.gitlab-ci.yml

+image: "ubuntu:latest"
+
+stages:
+  - build
+  - test
+
+before_script:
+  - apt update; apt-get install --yes --force-yes build-essential cmake
+
+build_cmake:
+  stage: build
+  script:
+    ./build_cmake_release.sh
+  artifacts:
+    paths:
+      - cmake-build-release/bin/
+
+run_tests:
+  stage: test
+  script:
+    ./build_cmake_release.sh; ./cmake-build-release/bin/tests
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