Port default_value_t to the Builder API

[epage]

With #3732, we now have a typed API. This means we can move the derive's default_value_t down into the builder API

This would supersede #2813

[epage]

This requires the ability to render a string for help.

I would suggest TypedValueParser gaining

fn display(&self) -> Option<String> {
    None
}

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