Implement PASERK serialization

[minus7]

Implement serialization and deserialization of v2/3/4 local/secret/public keys as well as key identifiers (lid/sid/pid).

I based this on @suttod's work (#3) and integrated the feedback you left there, @aidantwoods. There's still some things that probably should be improved, but I'm pushing this now to get some feedback as this has been sitting here for a month already and we're already using it in a few places.

[aidantwoods]

Hey! I just wanted to say thanks for the PR. I'd been meaning to pick this up myself, so really appreciated 🙂

Have had a pretty busy couple of weeks so apologies for not getting to this yet, I'll give this a proper review as soon as I can.

[minus7]

Hey, just a gentle reminder in case this went under

[Dexus]

@aidantwoods do you not had any time for this? :(