tree: add tree branch reference

allow multiple independent iterators for the same underlying tree

safety: not very good, but should work because original tree is kept in
Parser and branches are only used during its lifetime

src/block.rs

@@ -418,7 +418,7 @@ mod test {
     macro_rules! test_parse {
             ($src:expr $(,$($event:expr),* $(,)?)?) => {
                 let t = super::TreeParser::new($src).parse();
-                let actual = t.map(|ev| (ev.kind, ev.span.of($src))).collect::<Vec<_>>();
+                let actual = t.root().map(|ev| (ev.kind, ev.span.of($src))).collect::<Vec<_>>();
                 let expected = &[$($($event),*,)?];
                 assert_eq!(actual, expected, "\n\n{}\n\n", $src);
             };