Allow #[serde(default)] on tuple structs

[Mingun]

Right now you cannot place #[serde(default)] at tuple struct, although it is possible to mark individual fields as #[serde(default)]:

#[derive(Deserialize)]
struct Tuple(
  // possible, but make no sense in that case, because if
  // field will be set to default value that means that sequence
  // is exhausted and the next field will fail to deserialize
  #[serde(default)] u8,
  u8,
);
#[derive(Deserialize)]
#[serde(default)] // impossible, but make sense
struct Tuple(
  #[serde(default)] u8,
  u8,
);

This PR fixes that and also guaranties that tuple structs will always have a trail of defaulted non-skipped fields as soon as first defaulted field is appear.

[dtolnay]

Thanks!