factor out simple templates into a separate SimpleTemplateDir

there's no need to bloat TreehouseDir with that logic

src/vfs/html_canonicalize.rs

@@ -2,6 +2,7 @@ use core::fmt;
 
 use super::{Dir, Query, VPath};
 
+/// This Dir exists to serve as a compatibility layer for very old links that end with .html.
 pub struct HtmlCanonicalize<T> {
     inner: T,
 }