add getter for root of snapshot

src/lib.rs

@@ -317,6 +317,10 @@ impl<R> Snapshot<R> {
     fn read_raw(&self, range: FileRange) -> &[u8] {
         &self.map[range.as_range()]
     }
+
+    pub fn root(&self) -> FilePointer<R> {
+        self.root
+    }
 }
 
 pub struct Reader<R> {