Support desugaring union into struct for safe field access

[joshlf]

Credit to @kupiakos for this idea

Requirements

• Should be able to generate safe accessors
• Should be able to implement FromBytes/IntoBytes
• Also TryFromBytes fields? Safe field accessor is still safe, but now fallible
  • How do we tell the attribute which fields are only TryFromBytes?
• Should be able to codegen most of this based on a macro or proc macro attribute

Details

A macro or proc macro attribute that permits you to write a union. It is assumed that all fields are IntoBytes. It is assumed that all fields are FromBytes unless explicitly annotated as TryFromBytes.

#[zerocopy(union)]
union Foo {
    // No annotation => assumes this is FromBytes
    a: A,
    #[zerocopy(TryFromBytes)]
    b: B,
}

This desugars into a [u8; N] (wrapped in a type which guarantees the appropriate alignment) which provides safe accessor methods, e.g.:

#[repr(transparent)]
struct Foo(...);

impl Foo {
    // A: FromBytes
    fn a(&self) -> &A {
        
    }
    
    // B: TryFromBytes
    fn b(&self) -> Option<&B> {
        
    }
}

The internals could be represented, for example, as:

// This could live in `zerocopy::util::macro_util`
union UnionInner<T, Rest> {
    t: T,
    rest: Rest,
}

// Generated by the proc macro attribute
#[repr(transparent)]
struct Foo(AlignedBytes<UnionInner<A, UnionInner<B, C>>>);