Add a cargo workspace containing both crates (#321)

* Move Cargo.toml for exercises to exercises directory.

* Create a workspace with both exercises and i18n-helpers.

* Build in CI as well as testing.

* Binaries must have a main function.

* No need for workspaces configuration for caching anymore.

.github/workflows/setup-rust-cache/action.yml

@@ -7,7 +7,3 @@ runs:
   steps:
     - name: Setup Rust cache
       uses: Swatinem/rust-cache@v2
-      with:
-        workspaces: |
-          . -> target
-          i18n-helpers -> target