Rollup merge of #41449 - Eh2406:master, r=aturon

FromIterator and Extend Cow for String

This is a quick draft to start working on [#41351](https://github.com/rust-lang/rust/issues/41351).
I don't think I got the stable attributes correct, but it is good enuf to start a conversation.

src/libcollections/string.rs

@@ -1604,6 +1604,15 @@ impl FromIterator<String> for String {
     }
 }
 
+#[stable(feature = "herd_cows", since = "1.19.0")]
+impl<'a> FromIterator<Cow<'a, str>> for String {
+    fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
+        let mut buf = String::new();
+        buf.extend(iter);
+        buf
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Extend<char> for String {
     fn extend<I: IntoIterator<Item = char>>(&mut self, iter: I) {
@@ -1641,6 +1650,15 @@ impl Extend<String> for String {
     }
 }
 
+#[stable(feature = "herd_cows", since = "1.19.0")]
+impl<'a> Extend<Cow<'a, str>> for String {
+    fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I) {
+        for s in iter {
+            self.push_str(&s)
+        }
+    }
+}
+
 /// A convenience impl that delegates to the impl for `&str`
 #[unstable(feature = "pattern",
            reason = "API not fully fleshed out and ready to be stabilized",