Request for review - std.serialization (orange)

[Kagamin]

It's a pull parser? Hmm... how reordered fields are supposed to 
be handled? When the archiver is requested for a field, it will 
probably need to look ahead for the field in the entire message. 
Also arrays can be discontinuous both in xml and in pb. Also if 
the archiver is requested for a missing field, it may be a bad 
idea to return typeof(return).init as it will overwrite the 
default value for the field in the structure. Though, this may be 
a minor issue: field usually is missing because it's obsolete, 
but the serializer will spend time requesting missing fields.

As a schema-informed serialization, PB works better with 
specialized code, so it's better to provide a means for 
specialized serialization, where components will be tightly 
coupled, and the archiver will have full access to the serialized 
type and will be able to infer schema. Isn't serialization 
simpler when you have access to the type?