implement RSS feeds

crates/treehouse/src/state.rs

@@ -67,6 +67,7 @@ pub struct FileId(usize);
 pub struct Treehouse {
     pub files: Vec<File>,
     pub files_by_tree_path: HashMap<VPathBuf, FileId>,
+    pub feeds_by_name: HashMap<String, FileId>,
 
     pub tree: SemaTree,
     pub branches_by_named_id: HashMap<String, SemaBranchId>,
@@ -82,6 +83,7 @@ impl Treehouse {
         Self {
             files: vec![],
             files_by_tree_path: HashMap::new(),
+            feeds_by_name: HashMap::new(),
 
             tree: SemaTree::default(),
             branches_by_named_id: HashMap::new(),