Proper unit and newtype_variant support

[du-toit]

As of now, the library does not properly serialize or deserialize unit structs, unit variants, new type structs or new type variants:

fn serialize_unit_struct(
    self,
    _: &'static str,
) -> Result<Self::Ok, Self::Error> {
    Ok(())
}

fn serialize_unit_variant(
    self,
    _: &'static str,
    _: u32,
    _: &'static str,
) -> Result<Self::Ok, Self::Error> {
    Ok(())
}

fn serialize_newtype_struct<T: ?Sized>(
    self,
    _: &'static str,
    _: &T,
) -> Result<Self::Ok, Self::Error>
where
    T: serde::Serialize,
{
    Ok(())
}

fn serialize_newtype_variant<T: ?Sized>(
    self,
    _: &'static str,
    _: u32,
    _: &'static str,
    _: &T,
) -> Result<Self::Ok, Self::Error>
where
    T: serde::Serialize,
{
    Ok(())
}

This is considered a bug and should be fixed accordingly.

[du-toit]

#4 Fixes both serialize_unit_struct and serialize_newtype_struct but the other two will be fixed when we have enum support, so I'm keeping this issue open.