should be able to specify the type when initializing a container from an iterator

[tarakc02]

A lot of the Iterators.\* types have an eltype of Any. Don't know if it's worth doing something more fancy to determine the container type at construction (makes a big difference in performance), but at the very least a user should be able to specify it, e.g.:

julia> PureFun.Linked.List{Int}(1:10)

[tarakc02]

currently this won't really work, and as a workaround we use some awkward type inference stuff. keeping the issue open to rethink module designs...