Improve Serialize of adjacently tagged newtype variants

[dtolnay]

The existing implementation was unnecessarily complicated.

struct __AdjacentlyTagged<'__a> {
    data: (&'__a String,),
    phantom: _serde::export::PhantomData<AdjacentlyTagged>,
}
impl<'__a> _serde::Serialize for __AdjacentlyTagged<'__a> {
    fn serialize<__S>(
        &self,
        __serializer: __S,
    ) -> _serde::export::Result<__S::Ok, __S::Error>
    where
        __S: _serde::Serializer,
    {
        let (__field0,) = self.data;
        _serde::Serialize::serialize(__field0, __serializer)
    }
}
_serde::ser::SerializeStruct::serialize_field(
    &mut __struct,
    "content",
    &__AdjacentlyTagged {
        data: (__field0,),
        phantom: _serde::export::PhantomData::<AdjacentlyTagged>,
    },
)?;

Instead the new implementation does simply:

_serde::ser::SerializeStruct::serialize_field(
    &mut __struct,
    "content",
    __field0,
)?;

Fixes #1224.