conlang: Import items into scope with use!

grammar:
- Improve specification of `Path`
- Add `Use` and `UseTree` rules
- Add `Use` as a variant of ItemKind

cl-token:
- Add new keywords `use` and `as`

cl-ast:
- Add nodes for Use and UseTree
- Add new ItemKind for Use
- Implement traversal in Visit and Fold

cl-interpret:
- Mark ItemKind::Use with a todo

cl-parser:
- Update to match grammar

cl-typeck:
- Update to match changes in AST
- Mark UseTrees as NameCollectable and TypeResolvable, but leave as todos

compiler/cl-repl/examples/yaml.rs

@@ -207,6 +207,7 @@ pub mod yamlify {
                 ItemKind::Struct(f) => y.yaml(f),
                 ItemKind::Enum(f) => y.yaml(f),
                 ItemKind::Impl(f) => y.yaml(f),
+                ItemKind::Use(f) => y.yaml(f),
             };
         }
     }
@@ -322,6 +323,22 @@ pub mod yamlify {
             };
         }
     }
+    impl Yamlify for Use {
+        fn yaml(&self, y: &mut Yamler) {
+            let Self { tree } = self;
+            y.key("Use").yaml(tree);
+        }
+    }
+    impl Yamlify for UseTree {
+        fn yaml(&self, y: &mut Yamler) {
+            match self {
+                UseTree::Tree(path, tree) => y.pair("path", path).pair("tree", tree),
+                UseTree::Alias(path, name) => y.pair("path", path).pair("name", name),
+                UseTree::Path(path) => y.pair("path", path),
+                UseTree::Glob => y.value("Glob"),
+            };
+        }
+    }
     impl Yamlify for Block {
         fn yaml(&self, y: &mut Yamler) {
             let Self { stmts } = self;