The existing function, IsOneOf, is hard to use. Since the slice comes
before the rune, in parallelism with the other Is functions, the slice
is clumsy to build. This CL adds a nicer-signatured In function of
equivalent functionality (its implementation is identical) that's much
easier to use. Compare:
        unicode.IsOneOf([]*unicode.RangeTable{unicode.Letter, unicode.Number}, r)
        unicode.In(r, unicode.Letter, unicode.Number)

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/11672044