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-ast/src/ast.rs

@@ -99,6 +99,8 @@ pub enum ItemKind {
     Function(Function),
     /// An [implementation](Impl)
     Impl(Impl),
+    /// An [import](Use)
+    Use(Use),
 }
 
 /// An alias to another [Ty]
@@ -223,6 +225,21 @@ pub enum ImplKind {
     Trait { impl_trait: Path, for_type: Box<Ty> },
 }
 
+/// An import of nonlocal [Item]s
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct Use {
+    pub tree: UseTree,
+}
+
+/// A tree of [Item] imports
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum UseTree {
+    Tree(Path, Vec<UseTree>),
+    Alias(Path, Identifier),
+    Path(Path),
+    Glob,
+}
+
 /// A type expression
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Ty {