Create a type to iterate over data as if it was with known type

[zhiburt]

It's often used approach to index some batch of columns/rows.

E.g.

for entry in 1 .. table.count_rows() / Swim::LENGTH { 
  style.insert_horizontal_line(entry * Swim::LENGTH, HorizontalLine::inherit(Style::modern())); 
 } 

All these math tricks are better be hidden,
So we could have something like.

table.count_records::<Swim>() -> usize

Not sure about the interface but it's very useful thing if we could make it beautiful.

[zhiburt]

The kicker here is that we are losing type information after building table.
But sometimes it's useful to know Tabled::LENGTH
And maybe even type of construction Table::kv or Table::new depending on the found interface

[zhiburt]

TableIterator::from_table::<Type>(&table)