Port required_if_eq to the typed API

[epage]

With #3732, we now have a typed API. We should support typing in the rest of the API rather than relying on raw values.

[epage]

This requires the ability to compare values.

I would suggest TypedValueParser gaining

fn partial_eq(&self) -> Option<bool> {
    None
}

We would then provide it for all of the built-in value parsers. The question is if we should make PartialEq a requirement for the FromStr blanket impl.

See also #3807