Feature request: permit subenum to be "unit-only"

[BatmanAoD]

It would be nice to be able to extract just the variants from an enum as a sub-enum, ignoring tuple-like or struct-like data. So this:

#[repr(u32)]
#[subenum(Variants(unit_like))]
enum Foo {
    #[subenum(Variants)]
    SomeTuple(i32, String) = 37,
    #[subenum(Variants)]
    SomeStruct{
        data: String,
    } = 42,
}

...would generate something like this:

#[repr(u32)]
enum Variants {
    SomeTuple = 37,
    SomeStruct = 42,
}

impl TryFrom<Foo> for Variants {
    ...
}

Of course, Foo could not impl From<Variants> unless all the inner data implemented Default, and even then it might be preferable to have a more explicit impl Variants { pub fn as_foo_with_defaults(self) -> Foo { ... } }.