conlang: Update type checker

- cl-typeck: Add modules, intrinsic types, unify definition ids
- cl-ast: make attribute lists `Default`
- cl-structures: Add functions to iterate through a pool
- cl-repl: Create an example REPL for the type checker

cl-ast/src/lib.rs

@@ -37,7 +37,7 @@ pub struct File {
 }
 
 // Metadata decorators
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub struct Attrs {
     pub meta: Vec<Meta>,
 }