Auto merge of #14296 - Wafflespeanut:either, r=emilio

Some generic impls for Either<Length, T>

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We seem to be using `parse_non_negative` a lot for `LengthOrFoo` types, so it's better to have generic impls (now, and in the future) for such things (assuming that `Length` will always be in the first variant of `Either`).

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- [x] These changes do not require tests because it's a refactor

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