[racket-dev] provide expand-clause in racket/private/for.rkt

As far as I can see, exporting `expand-clause` is ok.

Ideally, I think it should be exported from a new
`racket/for-transform` library, instead of used directly from
`racket/private/for`. Also, `expand-for-clause` might be a better name.

At Fri, 6 Sep 2013 00:45:40 -0400, Stephen Chang wrote:
> Hi dev,
> 
> I would like to provide (for-syntax) the "expand-clause" function in
> racket/private/for.rkt. Would this cause any problems? Would anyone
> object to this?
> 
> I have an implementation of for/X in my generic-bind library that uses
> expand-clause and with it, the generic-bind ~for forms are as fast,
> sometimes a little faster, than racket's for/X in some preliminary
> testing. (Without access to expand-clause, sequence traversal is slow
> --- the current planet-available version of my library uses
> sequence-generate and is twice as slow.)
> 
> The "sometimes a little faster" is likely in part due to less
> error-checking, but my implementation passes all the for/X unit tests
> and so as is, it can reasonably be used in place of for/X.
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