Add `unit` to the definition of Stream
Refactor Stream primitives
Fix the type of `map` inherited by Enumerable
Implement new `removeNil` primitive
Implement Stream.scan
Add Enumerable._removeNil, without an initialValue
Implement Enumerable.concat
Move filter implementation up to Stream level
Implement Stream.take
Implement Stream.combinePrevious
Implement new `skipRepeats` primitive