Make List::new accept IntoIterator

[Valentin271]

Problem

Currently we have to write something like this to create a list.

List::new([ListItem::new("Item 1"), ListItem::new("Item 2"), ListItem::new("Item 3")]);

Would be nice and natural to be able to write

List::new(["Item 1", "Item 2", "Item 3"]);

Solution

Make List::new accept IntoIterator<Item = Into<ListItem>>.

Unfortunately this seems to be breaking bug, especially for empty lists as the compiler cannot infer ::Item's type

Alternatives

Leave as is

Additional context

This should probably be worked on after #669 is merged.