Duplicate visitors with yaserde_derive

[NiklasVousten]

If you have a simple struct with two fields of the same type and use yaserde_derive, two identical visitors will be generated for these.

Example:

#[derive(YaDeserialize)]
pub struct XmlStruct {
  #[yaserde]
  num1: u32,

  #[yaserde]
  num2: u32,
}

Using cargo expand shows that two visitors are generated

#[allow(non_snake_case, non_camel_case_types)]
struct __Visitor_Num1_;
impl<'de> ::yaserde::Visitor<'de> for __Visitor_Num1_ {
    type Value = u32;
    fn visit_u32(
        self,
        v: &str,
    ) -> ::std::result::Result<Self::Value, ::std::string::String> {
        u32::from_str(v).map_err(|e| e.to_string())
    }
}
#[allow(non_snake_case, non_camel_case_types)]
struct __Visitor_Num2_;
impl<'de> ::yaserde::Visitor<'de> for __Visitor_Num2_ {
    type Value = u32;
    fn visit_u32(
        self,
        v: &str,
    ) -> ::std::result::Result<Self::Value, ::std::string::String> {
        u32::from_str(v).map_err(|e| e.to_string())
    }
}

I believe the derive macro should only create implementations for the YaDeserialize trait of unknown types. Instead yaserde should have implementations for default visitors similar to serde.

Furthermore I think it would be good, if the primitives have an implementation for the YaDeserialize trait