early termination for `map'

[Andy Wingo <wingo@pobox.com>]

Hello,

If you call `map' or `for-each' with more than one list, our versions of
these operators will detect if the lists are of unequal length, and
throw an error in that case.

However, SRFI-1 has long provided an extension to this, to allow for
early termination when any of the lists runs out.  R6RS adopted this,
and it looks like R7RS will ratify that.

So perhaps it's time for us to change as well.

This would also allow us to get rid of the hack in srfi-1.c in which,
when and if GOOPS gets loaded, srfi-1 extends the `map' and `for-each'
primitive generics with its own early-termination code, which in effect
gives early termination to every `map' user, regardless of whether that
module has imported srfi-1 or goops.  Sometimes I think that Mikael put
the Oops in Goops for a reason ;-)

Andy
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