Auto merge of #25989 - jooert:implement_rfc839, r=Gankro

I had to use `impl<'a, V: Copy> Extend<(usize, &'a V)> for VecMap<V>` instead of `impl<'a, V: Copy> Extend<(&'a usize, &'a V)> for VecMap<V>` as that's what is needed for doing

```rust
let mut a = VecMap::new();
let b = VecMap::new();
b.insert(1, "foo");

a.extend(&b)
```

I can squash the commits after review.

r? @Gankro 

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libcollections/string.rs b/src/libcollections/string.rs
index 7edcf3d3c9a..76f7c9c6b17 100644
--- a/src/libcollections/string.rs
+++ b/src/libcollections/string.rs
@@ -793,6 +793,13 @@ impl Extend<char> for String {
     }
 }
 
+#[stable(feature = "extend_ref", since = "1.2.0")]
+impl<'a> Extend<&'a char> for String {
+    fn extend<I: IntoIterator<Item=&'a char>>(&mut self, iter: I) {
+        self.extend(iter.into_iter().cloned());
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a> Extend<&'a str> for String {
     fn extend<I: IntoIterator<Item=&'a str>>(&mut self, iterable: I) {