Remove redundant calls to up inside next

[dthume]

next currently calls up multiple times depending on the execution path, which in turn results in multiple calls to make-node. While this doesn't make a substantial difference to the performance of vector-zip, custom zippers in which make-node is more expensive benefit from removing the redundant calls.

[akhudek]

Sorry for being slow to respond. I haven't forgotten about this and will double check it and merge it soon.