Export measurev()

[MichaelChirico]

          since measure is exported, we should export measurev too. both are documented because they are meant to be used as the `measure.vars` argument to `melt`.

when I wrote measure/measurev, I was not aware of the issues created by breaking strings apart when translating messages.
so we should probably get rid of the group.desc argument, I will look at #6534

Originally posted by @tdhock in #6532 (comment)

measurev() is analogous to setorderv(), setkeyv(), setindexv().