Function derive_try_from_bytes_union 

pub(crate) fn derive_try_from_bytes_union(
    ast: &DeriveInput,
    unn: &DataUnion,
    top_level: Trait,
    zerocopy_crate: &Path,
) -> TokenStream

A union is TryFromBytes if:

• all of its fields are TryFromBytes and Immutable