Allow changing visibility of derived enum

[bgilbert]

I'd like to have a private enum as an implementation detail of my library, but provide a public method (on another type) that returns its kind. However, it seems enum-kinds always copies the visibility of the original enum.

For example:

#[derive(EnumKind)]
#[enum_kind(FooKind)]
enum Foo {
    A(u32),
    B(u64),
}

pub fn get_a_foo() -> FooKind {
    Foo::A(0).into()
}

This produces:

error[E0446]: private type `FooKind` in public interface
  --> src/lib.rs:36:1
   |
29 | #[derive(EnumKind)]
   |          -------- `FooKind` declared as private
...
36 | pub fn get_a_foo() -> FooKind {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't leak private type