Add support for more tokens served as enum variant identifiers

[Mingun]

This includes borrowed and owned variants of strings / bytes and all supported unsigned integers. Now the following tokens can be used as variant identifiers which is all supported by visitor implementations derived by serde_derive:

• Token::BorrowedStr
• Token::String
• Token::BorrowedBytes
• Token::ByteBuf
• Token::U8
• Token::U16
• Token::U64

This is needed for the my PR to serde which expands test suite by this test:

#[derive(Debug, PartialEq, Serialize, Deserialize)]
enum Enum {
    Tuple(u8, u8),
}

#[derive(Debug, PartialEq, Serialize, Deserialize)]
#[serde(tag = "tag")]
enum InternallyTagged {
    StructEnum { enum_: Enum },
}
//...
        let value = InternallyTagged::StructEnum {
            enum_: Enum::Tuple(4, 2),
        };
        assert_de_tokens(
            &value,
            &[
                Token::Struct {
                    name: "InternallyTagged",
                    len: 2,
                },
                Token::BorrowedStr("tag"),
                Token::BorrowedStr("StructEnum"),
                Token::BorrowedStr("enum_"),
                Token::Enum { name: "Enum" },
                Token::BorrowedStr("Tuple"), //<<< Impossible due to lack of support of BorrowedStr
                Token::Seq { len: Some(2) },
                Token::U8(4),
                Token::U8(2),
                Token::SeqEnd,
                Token::StructEnd,
            ],
        );

[Mingun]

Failed check is due to network problems on runner.

[Mingun]

Could this PR take attention? It is needed to improve test coverage in serde. @dtolnay, @oli-obk.