Remove Pair_impl class.
Remove Pair_impl class.

The Pair_impl class was only being used by the vector-based
implementation. Since we always use maps now, it shouldn't be
necessary.

upsoft/json-spirit,infinit/json-spirit,mickem/json-spirit,andmaj/json-spirit,infinit/json-spirit,mickem/json-spirit,upsoft/json-spirit,andmaj/json-spirit,sirikata/json-spirit,sirikata/json-spirit