删除 serde_type 模块，利用 `cfg_attr` 设置 serde 相关内容

[zjp-CN]

例子来源：pulldown-cmark

#[derive(Clone, Debug, PartialEq)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub enum Tag<'a> {
    #[cfg_attr(feature = "serde", serde(borrow))]
    FootnoteDefinition(CowStr<'a>)
}

[zjp-CN]

serde_type::day 移入 tests/file-day.rs，且增加 csv 测试样例