derive(CheckedBitPattern) should work on repr-stable enums with fields

[RReverser]

repr(C), repr(i*) and repr(u*) enums with fields have had a stable layout for FFI for about 5 years now, and recently arbitrary enum discriminants on enums with fields were stabilized as well.

It would be great to support them in the CheckedBitPattern derive so that examples from that issue could be casted to/from bytes as expected, e.g.:

#[derive(Clone, Copy, CheckedBitPattern)]
#[repr(u8)]
enum Foo {
    A = 0,
    B { a: u16 } = 1,
    C { a: u32, b: u16 } = 2,
}