Fold a lowered body, given the spans recorded while lowering it.
A span is one node's whole run, and it always follows its children's -- so
the widest span at a position is the node there, and the last instruction
in it is the head.
The NESTED lists arrive in completion order: deepest first, then left to
right, with each enclosing body after the bodies inside it. Taking them
from the BACK while walking each body's structural instructions right to
left visits them in exactly that order reversed, which is how each body
finds its own.