Allow deriving Zeroable for enums which are #[repr(u32, i8, etc..)]

[ARitz-Cracker]

If an enum is explicitly defined in a manner where "0" is a valid variant, and all the payloads of the 0 variant are zeroable, then it concludes that the enum is safely zeroable.

[ARitz-Cracker]

Additionally (though this may be a separate issue) an enum which is #[repr(u8)] and has all 256 variants explicitly defined can safely implement Pod.

[Lokathor]

That all sounds right to me.