Refactoring: prepare for addition of tuple formats.

Extract the remaining places which access tuple->data
directly into tuple_to_port.cc file, which will have full
access to all tuple formats and all destinations to which
a tuple may need to be converted.

In this file it'll be possible to perform a conversion
efficiently, at the same time, it will be the only place
of cross-dependency between all tuple formats and all
conversion destinations.