Remove serialize and deserialize impls for Result

[dtolnay]

I don't know of these impls ever being useful, and they lead to confusion when the user writes too many or too few ? or unwrap.

extern crate serde;
extern crate serde_json;

use serde::de::DeserializeOwned;

fn parse_t<T>() -> Result<T, String>
    where T: DeserializeOwned
{
    let j = r#" {} "#;

    // Compiles, but fails at runtime to deserialize `Result<T, String>`
    serde_json::from_str(j).unwrap()
}