Requirements on keys for bulk visitation are overly restrictive

Currently, bulk visitation requires that

>[f]or K = std::iterator_traits::value_type, either K [be] key_type or else Hash::is_transparent and Pred::is_transparent [be] valid member typedefs.

This precludes the scenario where the keys are implicitly convertible to `key_type`, which seems to be overly restrictive. The drawback of allowing this is that conversion from `std::iterator_traits::value_type` to `key_type` will happen more than once for each key (twice, to be precise).