Hello, SG10!

When P2494 <http://wg21.link/P2494> was being discussed in LEWG, there were
two competing directions for how to handle its feature test macro:

   1. introduce a new feature test macro that indicates this feature
   specifically; and
   2. bump __cpp_lib_ranges, since other features in flight for ranges have
   their own feature test macros.

LEWG requested that I ask this group for a recommendation. Personally I'm
leaning towards option number 1, since it feels cleaner to me.

Additionally, if this group recommends that I go with (1), I'd like
recommendations for what the name of the macro should be, because none of
the names that I'm coming up with are short enough to be usable, but also
long enough to be descriptive.

Thanks,
Michał