GPU backend

Adds GPU backend and "CUDA" feature to all the tests. As of MR creation, there were no forks in tested IRs so only vanilla non-parallel (single block, single thread) codegen has been tested.

.gitlab-ci.yml

@@ -2,4 +2,4 @@ test-job:
   stage: test
   script:
     - cargo test
-    - RUSTFLAGS="-C link-arg=-Wl,--no-as-needed" cargo build --features=cuda -vv
+    - cargo test --features=cuda