don't rearrange model decls during field reordering

After some upcoming changes to relax section ordering restrictions, this kind of
node reordering will have some undesirable side effects. In order to pre-empt
this, we take advantage of the fact that fields are only being reordered to get
more optimal offsets. Since the offset of state variables is stored within them
and not calculated dynamically, we can simply update their (explicit) offsets,
and leave their ordering as-is, neatly side stepping this upcoming issue.

Github: related to #202 "remove section ordering restriction"