vec: Fix size_hint() of reverse iterators

New lint `manual_split_once`

This is a WIP because it still needs to recognize more patterns. Currently handles:

```rust
s.splitn(2, ' ').next();
s.splitn(2, ' ').nth(0)
s.splitn(2, ' ').nth(1);
s.splitn(2, ' ').skip(0).next();
s.splitn(2, ' ').skip(1).next();
s.splitn(2, ' ').next_tuple(); // from itertools

// as well as `unwrap()` and `?` forms
```

Still to do:

```rust
let mut iter = s.splitn(2, ' ');
(iter.next().unwrap(), iter.next()?)

let mut iter = s.splitn(2, ' ');
let key = iter.next().unwrap();
let value = iter.next()?;
```

Suggestions on other patterns to check for would be useful. I've done a search on github for uses of `splitn`. Still have yet to actually look through the results.

There's also the question of whether the lint shouold trigger on all uses of `splitn` with two values, or only on recognized usages. The former could have false positives where it couldn't be replaced, but I'm not sure how common that would be.

changelog: Add lint `manual_split_once`