Use `Arc<[Line]>` instead of `Vec<Line>` in `BigText`.

[TauYip]

The point is to avoid redundant allocations when rendering BigText.
There can be two ways to do that:

• Use Arc<[Line]> instead of Vec<Line> to share the same text lines.
• Or try to impl Widget for &BigText.

[joshka]

Probably the second option, not the first - but also that for all the widgets that are readonly.