Make #[derive(Anything)] into sugar for #[derive_Anything]

This is a hack, but I don't think we can do much better as long as `derive` is
running at the syntax expansion phase.

If the custom_derive feature gate is enabled, this works with user-defined
traits and syntax extensions. Without the gate, you can't use e.g. #[derive_Clone]
directly, so this does not change the stable language.

This commit also cleans up the deriving code somewhat, and forbids some
previously-meaningless attribute syntax. For this reason it's technically a

    [breaking-change]

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