Alleviate box.unpack() limit of 8000 tuples

[Kostja Osipov]

Currently select_range() can only return 8000 tuples, because it puts these tuples into a Lua table, and then calls unpack() on the table.

Instead, teach LuaPort to send Lua tables which consist entirely from box.tuple instances as is, without repacking them into a tuple.
I.e. when we do send of a Lua table, check the type of the first element: if it is a scalar, send the table as if it was a tuple of fields, if it is a tuple, send it as if it was a result set of tuples.

[UNera (unera)]

  function bla ()
        local lst = {}
        local index = box.space[0].index[0]
        for d in index:iterator(box.index.GE) do
                 table.insert(lst, d)
        end
        return lst
    end,

Such code throws an exception: 12290: Return type 'userdata' is not supported in the binary protocol