Auto merge of #66577 - WaffleLapkin:iter_take_while_map, r=mark-simulcrum

Add `Iterator::map_while`

In `Iterator` trait there is `*_map` version of [`filter`] — [`filter_map`], however, there is no `*_map` version of [`take_while`], that can also be useful.

### Use cases
In my code, I've found that I need to iterate through iterator of `Option`s, stopping on the first `None`. So I've written code like this:
```rust
let arr = [Some(4), Some(10), None, Some(3)];
let mut iter = arr.iter()
    .take_while(|x| x.is_some())
    .map(|x| x.unwrap());

assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), Some(10));
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), None);
```
Thit code
1) isn't clean
2) In theory, can generate bad bytecode (I'm actually **not** sure, but I think that `unwrap` would generate additional branches with `panic!`)

The same code, but with `map_while` (in the original PR message it was named "take_while_map"):
```rust
let arr = [Some(4), Some(10), None, Some(3)];
let mut iter = arr.iter().map_while(std::convert::identity);
```

Also, `map_while` can be useful when converting something (as in [examples]).

[`filter`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter
[`filter_map`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter_map
[`take_while`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.take_while
[examples]: https://github.com/rust-lang/rust/compare/master...WaffleLapkin:iter_take_while_map?expand=1#diff-7e57917f962fe6ffdfba51e4955ad6acR1042

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