Adds Serialize with AsRef or ToString and Deserialize with FromStr

[wyatt-herkamp]

Adds

• Serialize #[serde(as_ref = "Type") Calls AsRef and serializes the value
• Serialize #[serde(to_string) Calls ToString and serializes the String
• Deserialize #[serde(from_str) Calls FromStr for deserialize.

[dtolnay]

Thanks for the PR!

The from_str and to_string part of this are a duplicate of #2017. I would prefer not to build that into this crate but they are available in serde_with. The implementation there is more powerful than the one in this PR because deserialization works for data formats that do not expose borrowed data, such as deserializing from a std::io::Read.

For as_ref, I would similarly prefer not to build that into this crate. Like to_string, this is substituting for a pretty trivial Serialize impl that one can go ahead and handwrite without needing macro support; I don't see the use case occurring commonly enough to warrant dedicating an attribute (and compile time) to.