Implement BytesMut::extend_from_within (#818)

tests/test_bytes.rs

@@ -607,6 +607,23 @@ fn extend_from_slice_mut() {
     }
 }
 
+#[test]
+fn extend_from_within_normal() {
+    let mut bytes = BytesMut::new();
+    bytes.extend_from_slice(&LONG[..23]);
+    bytes.extend_from_within(10..22);
+    bytes.extend_from_within(22..35);
+    assert_eq!(LONG[..], *bytes);
+}
+
+#[test]
+#[should_panic]
+fn extend_from_within_out_of_range() {
+    let mut bytes = BytesMut::new();
+    bytes.extend_from_slice(&LONG[..23]);
+    bytes.extend_from_within(23..=23);
+}
+
 #[test]
 fn extend_mut_from_bytes() {
     let mut bytes = BytesMut::with_capacity(0);