Warn that repr(packed) and repr(align) do not guarantee field order, and as a result do not guarantee size

[joshlf]

While this is already implied by the type layout docs, it's easy to miss since the type layout docs describe reprs as guaranteeing things about the layout, and the special case of repr(packed(1)) does guarantee size.

In particular, call out the following:

• A repr(packed(N)) struct's size may change depending on field order
  • As a special case, a repr(packed(1)) or repr(packed) struct's size does not depend on field order
• A repr(align(N)) struct's size may change depending on field order
• In each of these cases, suggest using repr(C) to guarantee field order if guaranteed size is important