Any iterator whose item is convertible into `Row` can now be
collected into a `Table`.
Where previously, `Table::new` accepted `IntoIterator<Item = Row>`, it
now accepts `IntoIterator<Item: Into<Row>>`.
BREAKING CHANGE:
The compiler can no longer infer the element type of the container
passed to `Table::new()`. For example, `Table::new(vec![], widths)`
will no longer compile, as the type of `vec![]` can no longer be
inferred.
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