Soundness: Reads from padding bytes of unions in generated Debug impl

[Manishearth]

educe/src/trait_handlers/debug/debug_union.rs

Lines 53 to 59 in bc05181

	let size = ::core::mem::size_of::<Self>();
	let data = unsafe { ::core::slice::from_raw_parts(self as *const Self as *const u8, size) };
	builder.field(&data);
	builder.finish()

This is unsound: there is no guarantee those bytes are readable.