derive(Zeroable) on fieldful enums and repr(C) enums

[zachs18]

Closes #230.

Allows deriving Zeroable on enums where:

1. The enum is #[repr(Int)], #[repr(C)], or #[repr(C, Int)],
2. There is a variant with discriminant 0,
3. The variant with discriminant 0 is either fieldless, or all of its fields are Zeroable.

Also allows using the "perfect derive with additional bounds" from #196 for Zeroable on enums, where the fields for the "perfect derive" are from the variant with discriminant 0 (see the MyOption example on line 189 of derive/src/lib.rs).

Internal changes:

• VariantDiscriminantIter now also gives the &'a Variant associated with the discriminant.
• the comment on line 338 of derive/src/traits.rs kept getting mangled by cargo fmt, so I put it on its own line.
• In derive_marker_trait_inner: let predicates = ... moved to after let fields = ..., since computing fields now requires accessing input, but predicates borrows it mutably. Does not change semantics.
• generate_fields_are_trait and some other helper functions now take an Option<&Variant> to operate on if they otherwise don't support enums.
• some cargo fmt
• drive-by clarification/typo fixes (e.g. renaming discriminantor -> discriminant)

[Lokathor]

Sorry about the delay, bytemuck_derive-1.8.0 should be out now