Refactor signed object constructors

[bifurcation]

Signed objects, especially GroupInfo and KeyPackage, are used in basically two ways:

1. Instantiate => sign => serialize
2. Deserialize => verify => instantiate

Given that, it seems like we be able should have two constructors for these objects:

1. A "signing constructor" of the form Thing(fields..., sig_priv)
2. A "verifying constructor" of the form Thing(const bytes&, verify_data...) (where verify_data would include things like a tree from which the public key would be taken for GroupInfo)

The type might still need to be default-constructible to facilitate deserialization. Or it might be possible to define tls::get<T>(tls::ostream&), and then do:

Thing::Thing(tls::istream& str)
  : field1(tls::get<FieldType1>(str))
  , field2(tls::get<FieldType2>(str))
  , ...
{
  // verify signature
}

That would be a bit more boilerplate than TLS_SERIALIZABLE, but might be worth it for a clearer API.