This allows passing an array, slice or Vec of constraints, which is more
ergonomic than requiring this to always be a slice.
The following calls now all succeed:
```rust
Table::new(rows).widths([Constraint::Length(5), Constraint::Length(5)]);
Table::new(rows).widths(&[Constraint::Length(5), Constraint::Length(5)]);
// widths could also be computed at runtime
let widths = vec![Constraint::Length(5), Constraint::Length(5)];
Table::new(rows).widths(widths.clone());
Table::new(rows).widths(&widths);
```
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