Add Monoid instances for the certificate chain types.

Vincent, I added Monoid instances for `CertificateChain` and `CertificateChainRaw`. I originally was thinking of using this for handing chain certificates in TLS, but now I see that the chain from each file needs its own key. In any case, the `Monoid` instances still make sense and are natural. They at least allow you to use things like `Foldable.foldMap` when reading a certificate file.

So, in the end I don't really need this. But I already did the PR, so I'm sending it to you and leaving it up to you whether you want to merge it.

Regards,
Yitz