Make StdError identical to core::error::Error on feature="unstable"

[dtolnay]

Without this, a user might be forced to implement 2 different duplicate traits when using serde in no-std mode.

impl serde::ser::StdError for MySerError {...}

impl core::error::Error for MySerError {...}

impl serde::Serializer for MySerializer {
    type Error = MySerError;
    ...
}