`Extend<&str>` and `FromIterator<&str>` for String


We can now implement Extend<&str> and FromIterator<&str> in parallel with the existing char variants of these traits for String.

We should probably do this since we already use &str as particles of a string – see the graphemes iterator. Does it need an RFC?


This is not a language change,so I would say it does not need an RFC.