The new min_category is similar to minimum_category but accepts variable
number of iterator categories on input instead of just two, and also does
not depend on Boost.MPL for lambda placeholders. The existing minimum_category
trait has been reimplemented in terms of min_category and deprecated.
We don't yet emit deprecation warnings as there is still code that uses
the old trait which we first need to update. Eventually, minimum_category
will emit warnings and will be removed.
