7741: Add convert_for_to_iter_for_each assist r=mattyhall a=mattyhall
Implements one direction of #7681
I wonder if this tries to guess too much at the right thing here. A common pattern is:
```rust
let col = vec![1, 2, 3];
for v in &mut col {
*v *= 2;
}
// equivalent to:
col.iter_mut().for_each(|v| *v *= 2);
```
I've tried to detect this case by checking if the expression after the `in` is a (mutable) reference and if not inserting iter()/iter_mut(). This is just a convention used in the stdlib however, so could sometimes be wrong. I'd be happy to make an improvement for this, but not sure what would be best. A few options spring to mind:
1. Only allow this for types that are known to have iter/iter_mut (ie stdlib types)
2. Try to check if iter/iter_mut exists and they return the right iterator type
3. Don't try to do this and just add `.into_iter()` to whatever is after `in`
Co-authored-by: Matt Hall <matthew@quickbeam.me.uk>
