- 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
- syntax is now (generator formals body ...+)
- add `generator?'
- remove common run-time code from the `generator' macro expansion
- doc fixes
- start test suite
