Use: add alias use foo as bar form

src/ast/fold.rs

@@ -131,6 +131,7 @@ impl<A: Annotation> Foldable<A> for Use {
         Ok(match self {
             Self::Glob => Self::Glob,
             Self::Name(name) => Self::Name(name),
+            Self::Alias(name, alias) => Self::Alias(name, alias),
             Self::Path(name, rest) => Self::Path(name, rest.fold_in(folder)?),
             Self::Tree(items) => Self::Tree(items.fold_in(folder)?),
         })