This PR adds a `Sized` bound to the `StyleHelpers` trait.

All of the individual methods on this trait already had a `Self: Sized`
bound, so moving it up to the trait level will make it so we don't have
to repeat ourselves so much.

There's an open question of whether we can hoist the `Sized` bound to
`Styleable`, but it's possible there are cases where we'd want to have a
`Styleable` trait object.

Release Notes:

- N/A