Add serde(explicit_tags) attribute

[darkwater]

Closes #2763.

• Serialization
• Deserialization
• Externally tagged
• Any other tagging methods (do all apply? I don't think postcard supports all)
• Proper error handling
• Tests

Does this approach look good? I'm not great with proc macros and this is my first time contributing to serde.

This makes the following work:

use serde::{Deserialize, Serialize};

#[derive(Debug, PartialEq, Serialize, Deserialize)]
#[repr(i8)]
#[serde(explicit_tags)]
pub enum Foo {
    Zero,
    One,
    Five = 5,
    Six,
    Ten(u32) = 10,
    Twenty { value: u32 } = 20,
    // Invalid = -1,
    // error:    ^^ unsupported expression for enum discriminant (Serialize)
    //
    // Deserialize panics with "message: unsupported discriminant expression"
}

#[cfg(test)]
mod tests {
    use super::*;

    fn t(value: Foo, expected: &[u8]) {
        let serialized = postcard::to_allocvec(&value).unwrap();
        assert_eq!(serialized, expected);

        let deserialized = postcard::from_bytes::<Foo>(&serialized);
        assert_eq!(deserialized, Ok(value));
    }

    #[test]
    fn test_name() {
        t(Foo::Zero, &[0]);
        t(Foo::One, &[1]);
        t(Foo::Five, &[5]);
        t(Foo::Six, &[6]);
        t(Foo::Ten(42), &[10, 42]);
        t(Foo::Twenty { value: 84 }, &[20, 84]);
    }
}