Switch back to automatic Debug derive for Header struct

[nicholasbishop]

The Header Debug implementation was accidentally printing the size
field for the signature. Rather than just fix that field, switch the
whole thing back to using derive(Debug). This adds in the _reserved
field, which probably doesn't matter much either way but might as well
show the whole struct when debug printing.

[GabrielMajeri]

I've also discovered the Derivative crate which would allow us to use a custom Debug trait derivation with support for skipping certain fields. Would it be worth adding it as a dependency and using it?

[nicholasbishop]

I've also discovered the Derivative crate which would allow us to use a custom Debug trait derivation with support for skipping certain fields. Would it be worth adding it as a dependency and using it?

I think in this case I would mildly prefer to continue printing the reserved field, because the spec mentions that it must be set to zero. So I see this as a bit different from a padding field, which could be safely set to any value. If you debug print the header and see the reserved field is not all zeros that could be useful info.

[GabrielMajeri]

Got it. LGTM then!