Add `extend_next` method to extendable structs

[Rua]

Like the extend method of many standard containers, this would take an iterator of extension structs and add them all:

pub fn extend_next(
    self,
    next: impl IntoIterator<Item = &'a mut dyn ExtendsWhatever>,
) -> Self

[Ralith]

That seems like a lot of extra generated code for something that could be addressed with a three-line for loop on the caller's side.