Don't put an extra seperator element at the end of the stream. Brings
it in-line with the list version (`add-between') and sequence
version (`sequence-add-between'). Includes a test.
- rename old `racket/stream' to `racket/sequence', and adjust
to avoid stateful iterations
- add a new `racket/stream' library with a `strean-cons' that is based
on SRFI 41
- adjust `for' and some sequence constructors like `in-range' to
work more directly with streams
