Make the Header lifetime more readable

[chifflier]

Hi,

Thank you for the PR. However, I'm not sure setting the Header lifetime to the input's is required: in those functions, the header is only used, not stored, so there are situations where the header could live less time than the input.

I think that if you want to clarify the lifetime, then adding an explicit second lifetime (for ex 'hdr), different from 'a, would be the correct way.

[chifflier]

If you look at the CI annotation (failure in doc tests at src/der/parser.rs#L335), you'll see the lifetime problem explained above in parse_der_implicit, parse_ber_implicit and others: the Header would now be forced to have the lifetime of the input bytes.

Now, I do not think the solution is to tie the lifetimes together, there is no real reason to do so, and it add constraints to the input.