Auto merge of #478 - serde-rs:visitor, r=oli-obk
Auto merge of #478 - serde-rs:visitor, r=oli-obk

Simplify generated visitors

Neither `__FieldVisitor` nor `__Visitor` need the `__D: Deserializer` type parameter.

serde-rs/serde,oli-obk/rust-serde,erickt/serde