factor out simple templates into a separate SimpleTemplateDir

there's no need to bloat TreehouseDir with that logic

src/vfs/overlay.rs

@@ -2,6 +2,8 @@ use std::fmt;
 
 use tracing::instrument;
 
+use crate::vfs::ToDynDir;
+
 use super::{entries, Dir, DynDir, Entries, Query, VPath, VPathBuf};
 
 pub struct Overlay {
@@ -38,3 +40,18 @@ impl fmt::Debug for Overlay {
         write!(f, "Overlay({:?}, {:?})", self.base, self.overlay)
     }
 }
+
+pub fn layered_dir(layers: &[DynDir]) -> DynDir {
+    match layers {
+        [] => ().to_dyn(),
+        [dir] => dir.clone(),
+        [left, right] => Overlay::new(left.clone(), right.clone()).to_dyn(),
+        [left, right, rest @ ..] => {
+            let mut overlay = Overlay::new(left.clone(), right.clone());
+            for dir in rest {
+                overlay = Overlay::new(overlay.to_dyn(), dir.clone());
+            }
+            overlay.to_dyn()
+        }
+    }
+}