x509-cert: support creating Certificate without additional variables

[lumag]

x509-cert's Certificate struct can not be easily created in a separate function, as creating it requires several intermediate temporary variables, which are borrowed rather than being moved to the created struct. E.g. my first attempt stopped at borrowing signature, basic_contraint (for Extension's extn_value) and public key DER. I can probably solve the PK part by using newly implemented TryFrom, but for the rest additional work/suggestions would be required. E.g. it would be nice to allow using BitString in addition to BitStringRef for the signature field.

[tarcieri]

I've definitely been meaning to add a certificate builder type, as it's a blocker for migrating the yubhism crate (which would hopefully free up the x509 crate so we could start using that name).

I might have some time this weekend to look at that.

[lumag]

This would be great!

[baloo]

Could we close this one?

[tarcieri]

Fixed by #764