Support for generic fields in structs

[vilunov]

I need this one to support <xs:any/> XSD elements, so that the callers might decide what they expect themselves. I'm not yet sure how this should look in final form, as there may be many anys, submitting this PR as a general idea.

TODOs:

• support for parameters in struct serialization
• support for parameters in struct deserialization
• support for parameters in enums
• instead of requiring : YaSerialize at type def level, add it in macro result at trait impl level