Initially tuple_field_* getters were placed in tuple_format.h to avoid
including tuple_format.h in tuple.h. Now we include tuple_format.h
in tuple.h anyway, so move the code where it belongs. Besides, there
were a bunch of new getters added to tuple.h since then, so the code
has rotten a bit.

This is a preparation for an overhaul of tuple_field_* getters naming.