Complete enum derives with getters which return Option<&T>

[MTCoster]

I'm submitting this as an issue to gather feedback; I'm happy to do the work on an implementation if this is a desired feature.

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Currently I can use a combination of IsVariant and Unwrap to build an Option<T> for a specific variant of an enum.

It would be nice to be able to derive a getter which combines this functionality. For instance:

#[derive(GetVariant)]
enum MyEnum<A, B> {
    VarA(A),
    VarB(B),
}

Becomes:

enum MyEnum<A, B> {
    VarA(A),
    VarB(B),
}

impl <A, B> MyEnum<A, B> {
    fn var_a(&self) -> Option<&A> {
        match self {
            Self::VarA(var_a) => Some(var_a),
            _ => None,
        }
    }

    fn var_b(&self) -> Option<&B> {
        match self {
            Self::VarB(var_b) => Some(var_b),
            _ => None,
        }
    }
}

This somewhat mimics the implementation of .ok() and .err() on std's Result<T, E>, but those return Option<T> rather than Option<&T>. In that instance, this distinction doesn't matter as first convert to a Result<&T, &E> with .as_ref().