Rewrite serialization to use reflection

[erickt]

We need to use a syntax extension to automatically build serializers for records, structs, and enums. Instead, we should switch over to using the visitor-based reflection facilities.

[nikomatsakis]

I am not sure this is a good idea. Some types must have specialized serialization. A deriving-based solution may be more appropriate. (auto_serialize is essentially deriving, of course)

[bstrie]

erickt doesn't think this is still relevant.