`derive(IntoBytes)` accepts enums with padding

`derive(IntoBytes)` erroneously accepts `repr(C/int, align(X))` enums with static padding; e.g.

```rust
#[derive(zerocopy::IntoBytes)]
#[repr(u8, align(2))]
enum HasAlign {
    A
}
```

Fix in progress in #3064. Bug confirmed to be present as far back as 0.7.35 (the earliest un-yanked version of `zerocopy` in the 0.7 train).