Generic `join_with` function

[fpsvogel]

In a Zulip conversation, @rtfeldman suggested a List.join_with to complement the existing Str.join_with:

Str.join_with : List(Str), Str -> Str

List.join_with : List(item), item -> item
    where [item.join_with : List(item), item -> item]

The immediate context was me wanting to call join_with on a List, which is currently impossible since join_with is a Str function:

my_list.join_with(" ") # missing method error!

[fpsvogel]

Similar additions to List would be useful for the Str.from_utf* functions too, since they also operate on a List.

[rtfeldman]

This exists now!