Add EnumDiscriminants Trait and related Macro impl

[vpochapuis]

This PR has for goal to implement and resolve #376 as well as #176

[mohe2015]

Tested my crate with the new changes.

[yescallop]

I should note that these changes break the following user code:

#[derive(EnumDiscriminants)]
#[strum_discriminants(name(PrivateDiscriminants), vis(pub(self)))]
pub enum PublicEnum {
    Variant0(bool),
    Variant1 { a: bool },
}

The error message is:

error[E0446]: private type `PrivateDiscriminants` in public interface

This is because the IntoDiscriminant impl of PublicEnum leaks the private PrivateDiscriminants type. Should we maybe exclude the IntoDiscriminant impl when the parent enum is public and the generated enum is private?