impl ~for with sequence transformers instead of sequence-generate

- need expand-clause and make-sequence exported from racket/private/for.rkt
update benchmark times in notes
~for now as fast (or faster) than racket for

changes to ~for/common:
- dropped use of call-with-values
- dropped one-accum distinction
- add #:result option to specify number of values produced by body
  when it is different from accums
update timing in notes
minor edit: require expand-clause
require expand-for-clause from syntax/for-transform
syntax/for-transform -> syntax/unsafe/for-transform