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.

src/exercises/day-2/points-polygons.rs

@@ -222,3 +222,5 @@ mod tests {
     }
 }
 // ANCHOR_END: unit-tests
+
+fn main() {}

src/exercises/day-2/solutions-morning.md

@@ -6,7 +6,4 @@
 
 ```rust
 {{#include points-polygons.rs}}
-
-#[allow(dead_code)]
-fn main() {}
 ```

src/exercises/day-2/strings-iterators.rs

@@ -68,3 +68,5 @@ fn test_matches_with_wildcard() {
     ));
 }
 // ANCHOR_END: unit-tests
+
+fn main() {}