Bump to current Rust edition 2021.

Cargo.toml

@@ -1,5 +1,6 @@
 [package]
 name = "ego-tree"
+edition = "2021"
 version = "0.7.0"
 description = "Vec-backed ID-tree"
 keywords = ["tree", "vec", "id", "index"]

src/iter.rs

@@ -1,7 +1,7 @@
 use std::ops::Range;
 use std::{slice, vec};
 
-use {Node, NodeId, NodeRef, Tree};
+use crate::{Node, NodeId, NodeRef, Tree};
 
 /// Iterator that moves out of a tree in insert order.
 #[derive(Debug)]

src/lib.rs

@@ -724,7 +724,7 @@ macro_rules! tree {
 
 impl<T: Debug> Debug for Tree<T> {
     fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
-        use iter::Edge;
+        use crate::iter::Edge;
         if f.alternate() {
             write!(f, "Tree {{")?;
             for edge in self.root().traverse() {
@@ -760,8 +760,8 @@ mod display;
 
 impl<T: Display> Display for Tree<T> {
     fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
-        use display::Indentation;
-        use iter::Edge;
+        use crate::display::Indentation;
+        use crate::iter::Edge;
 
         let mut indent: Indentation = Indentation::new(true);