Phantom types use repr(C), making them not "trivial" for repr(transparent) purposes

[RalfJung]

This crate interacts with rust-lang/rust#78586 in a suboptimal way:

#[phantom]
struct MyPhantom<T: ?Sized>;

#[repr(transparent)]
struct MyWrapper<T>(T, MyPhantom<T>);

The MyWrapper declaration triggers a FCW (likely to become a hard error soon: rust-lang/rust#155299) because MyPhantom contains a repr(C) type and repr(C) is an ABI-relevant marker on the type which means we cannot in general promise that it can just be ignored.