Implement SliceConcatExt for any iterable of string slices, not just Slice and Vec

Currently, these work (they both return "foo foo foo", as you'd expect):

["foo"; 3].connect(" ")
vec!["foo"; 3].connect(" ")

But these do not:

// type `collections::linked_list::LinkedList<&str>` does not implement any method in scope named `connect`
LinkedList::from_iter(repeat("foo").take(3)).connect(" ")
// type `core::iter::Take<core::iter::Repeat<&str>>` does not implement any method in scope named `connect`
repeat("foo").take(3).connect(" ")

Might std::slice::SliceConcatExt, be implemented for collections of and iterators over &strs?

Start with std::slice::SliceConcatExt and compare its current Slice and Vec behavior with the LinkedList and iterator examples in the issue. Determine whether extending connect to iterable collections and iterators over &str is feasible, and clarify the API scope before proposing the change.