padding/alignment doesn't work on fields of unspecific offset

[Shinmera]

See https://github.com/Shinmera/binary-structures/blob/main/protocol.lisp#L717

The issue is that padding and alignment are merged into one and not retained as separate fields, so the runtime has no idea how to untangle them and restore the alignment.

[Shinmera]

This is also related to #5, which would need yet another separate field. We can't push the alignment into the next slot, as it might be a tail slot, or have less strict alignment requirements that would conflict with the padding.