rollup merge of #19626: bluss/string-extend-str

rollup merge of #19626: bluss/string-extend-str

Strings iterate to both char and &str, so it is natural it can also be extended or collected from an iterator of &str.

Apart from the trait implementations, `Extend<char>` is updated to use the iterator size hint, and the test added tests both the char and the &str versions of Extend and FromIterator.