Share deserialize_with wrappers between visit_seq and visit_map

[dtolnay]

Currently a struct with #[serde(deserialize_with="...")] on a field will generate two identical wrappers to call the deserialize_with function - one in visit_seq and one in visit_map. We should share the same wrapper to speed up compilation of the generated code.

[dtolnay]

I don't think this is worth the complexity it would require in serde_codegen.