* 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.
* Extract common build steps to composite actions
This allows us to repeat ourselves less across the different jobs.
I also tested using a “reusable workflow” to factor out the common
steps. However, this starts a separate job without a shared
filesystem, which in turn requires us to upload/download artifacts
when we want to use them in several jobs. The artifacts are downloaded
one-by-one and this adds delays and extra steps to all jobs.
* Move Rust cache setup to its own build step
This made it easy to consistently setup the caching of our nested
projects via the “workspacs” config key.
