Support Direct Conversion Between Subenums Without Converting to Parent

[itotallyrock]

Right now to convert two different subenum types to each other you must do the following

#[subenum(A, B)]
enum Foo {
    #[subenum(A, B)
    First,
    #[subenum(A, B)
    Second,
    Third,
}

// Current
let a_version: A = A::First;
let b_version: B = B::try_from(Foo::from(a_version))?;

// Desired
let a_version: A = A::First;
let b_version: B = B::try_from(a_version)?;

This extra step of converting to Foo is likely optimized away, but I think its more obvious what is happening with only one conversion.