Provide a non-owning constructor for Issuer

[p-avital]

Hey rcgen folks!

I must say I really like what you've done with the new API changes, but I'm really missing a way to sign certificates without giving the issuer ownership of the KeyPair (especially frustrating since it's not Clone).

Maybe I missed an alternative (maybe there's a newtype around a reference that directly impls SigningKey), but this seems like a straight forward enough change.

Do keep in mind that if you accept this PR, you're probably locking yourself into having these MaybeOwned in the internal type representation (just thought I'd warn you :) ).

Note: feel free to reject this PR if you don't want that extra surface, I'm gonna newtype &KeyPair to get that copy-less use in my project :)

[djc]

(You might also be interested in #363.)

[djc]

Thanks! Can you squash your commits, please?

[p-avital]

Gah!
I have no way to do that without from GH (I didn't actually download the repo for such a simple PR) D:
Any chance you can use the Squash & Merge option on GH? You should have it if you unfold the merge button :)

[djc]

Gah! I have no way to do that without from GH (I didn't actually download the repo for such a simple PR) D: Any chance you can use the Squash & Merge option on GH? You should have it if you unfold the merge button :)

We normally use the merge queue, which doesn't make this easy, but we can bypass that if squashing is too hard for you.

[p-avital]

Aaaand squashed :)

[est31]

Thanks!

[cpu]

Thank you :-)