Fix compilation errors for the newly introduce test & add type inference.

Tests are basically functions for which the return type should unify with bool. In principle, the type checker could also check that a test function has no arguments but, a test function with arguments wouldn't parse in the first place; feels a bit hacky but it works when considering the pipeline as a whole.

  Note that the code generation is still to be done.

crates/project/src/lib.rs

@@ -336,6 +336,9 @@ impl Project {
                             func,
                         );
                     }
+                    Definition::Test(_) => {
+                        todo!()
+                    }
                     Definition::TypeAlias(ta) => {
                         type_aliases.insert((module.name.clone(), ta.alias.clone()), ta);
                     }