Replace direct access to tuple->data in obvious cases
tuple-access: code review fixes

Two significant changes (both arguable, since this patch is about
taste by a large measure):

1) Split tuple_seek() into two: tuple_rewind()
and tuple_seek().

tuple_rewind() simply initializes the iterator and
sets it to 'before-first' position.
tuple_seek() returns a field, and sets the iterator
to the position after the returned field.

Previosly there was only tuple_seek(), which required
tuple_next() to get the field, and it was confusing whether
or not tuple_next() after tuple_seek() is required.
Indeed, the very code I chagned sometimes would
call tuple_next() after tuple_seek() and sometimes would not.

2) Move the return value of the iterator outside
of iteration state.

The advantage of "binding" the out parameters of the iterator to
iterator state is that you bind once, and don't need to supply out
parameters into every call to tuple_next() function. It also makes
it easy to add extra out parameters for bind in the future (field
data type, flags, etc).

This makes "binding" technique popular in database driver
APIs. Binding makes API more stable, which is also
important in drivers.

In this case, however, binding adds for extra lines of code
and (possibly) few extra instructions. Plus, since
this is an internal API which we can change any day,
its stability is not as important. So it seems that for
now we're better of with an API which is a bit
more concise/efficient, and can switch to using binding later,
if needed.