This adds `FromIterator` impls for `Line` and `Text` that allow creating
`Line` and `Text` instances from iterators of `Span` and `Line`
instances, respectively.
```rust
let line = Line::from_iter(vec!["Hello".blue(), " world!".green()]);
let line: Line = iter::once("Hello".blue())
.chain(iter::once(" world!".green()))
.collect();
let text = Text::from_iter(vec!["The first line", "The second line"]);
let text: Text = iter::once("The first line")
.chain(iter::once("The second line"))
.collect();
```
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