Fix some things with builtin derives

 1. Err on unions on derive where it's required.
 2. Err on `#[derive(Default)]` on enums without `#[default]` variant.
 3. Don't add where bounds `T: Default` when expanding `Default` on enums (structs need that, enums not).

Also, because I was annoyed by that, in minicore, add a way to filter on multiple flags in the line-filter (`// :`). This is required for the `Debug` and `Hash` derives, because the derive should be in the prelude but the trait not.

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