Sequence.map and where should use new this.type instead of []

[kmarekspartz]

See f8a9d7f#commitcomment-5391488

[kmarekspartz]

This wouldn't work for Ranges, as map and filter would have to be written differently anyway. Perhaps FilteredRange and MappedRange classes which takes a function and a range, but still is a Range? Not sure about that.

[munificent]

Instead of new type, we could define a different create method that List and Range implement and both would return a new list. (Or we could just put that on Sequence.)

But that feels a bit strange. Let's sit on this for now. We've probably got more pressing issues than the corners of subclassing collections.

[munificent]

Old bug is old. :) .map() and .where() return custom lazy sequence types now.