Derive FromRefSelf and FromRefMutSelf

[ModProg]

This is something the standard library does, for example for String: https://doc.rust-lang.org/src/alloc/string.rs.html#2645

Basically, the derive would just wrap a Clone call. This allows functions taking Into<T> to also take &T.

impl From<&mut Self> for Something {
    fn from(this: &mut Self) -> Self {
        this.clone()
    }
}

impl From<&Self> for Something {
    fn from(this: &Self) -> Self {
        this.clone()
    }
}

[tyranron]

I'd like to see this rather as derive(From) customization with attributes, rather than separate macros.