Allow PNI in PendingMember

[rubdos]

Should fix #328

The other member decryption functions are ACI-only (which makes sense). I double-checked this.

@gferon, @boxdot, I have removed Serialize and Deserialize bounds on some structs. These were in place because Presage stores those structs directly in its database, but I don't think that is correct behavior. libsignal-service exposes those structs without any forward or backward compatibility guarantees, as illustrated in here: the uuid field is dropped in favor of the service_id field. Migration should be handled by the storage backend. Same reasoning goes for #324.

[rubdos]

Adding some more convenience methods and refactoring to this, while I'm implementing the changes in WF.

[gferon]

exposes those structs without any forward or backward compatibility guarantees

I just realized that while this statement is true, it's not really a big deal for data that's transient or can be re-fetched when invalidated (which is the case for groups).

[rubdos]

I just realized that while this statement is true, it's not really a big deal for data that's transient or can be re-fetched when invalidated (which is the case for groups).

Sure, but a big point of Signal is to rely as little as possible on state stored on the server. I don't think a "low-level" library like lss should facilitate that kind of "unorthodox behavior".