We support deriving AsBytes on generic repr(transparent) structs because we don't need to explicitly check for padding, and so we don't need to emit code that's incompatible with generic type arguments. We ought to be able to use the same approach to support generic repr(packed) structs as well.

EDIT: Looks like we do support them, but there are no tests for that support, and our error messages imply that we don't support them. We should clean that up.

cc @djkoloski