Any iterator whose item is convertible into `Line` can now be
collected into `Tabs`.
In addition, where previously `Tabs::new` required a `Vec`, it can now
accept any object that implements `IntoIterator` with an item type
implementing `Into<Line>`.
BREAKING CHANGE:
Calls to `Tabs::new()` whose argument is collected from an iterator
will no longer compile. For example,
`Tabs::new(["a","b"].into_iter().collect())` will no longer compile,
because the return type of `.collect()` can no longer be inferred to
be a `Vec<_>`.
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