box: introduce port_c

Port_c is a new descendant of struct port. It is used now for
public C functions to store their result. Currently they can
return only a tuple, but it will change soon, they will be able to
return arbitrary MessagePack.

Port_tuple is not removed, because still is used for box_select(),
for functional indexes, and in SQL as a base for port_sql.
Although that may be changed later. Functional indexes really need
only a single MessagePack object from their function. While
box_select() working via port_tuple or port_c didn't show any
significant difference during micro benchmarks.

Part of #4641