Tree transformer that duplicates code and at the same time creates
fresh symbols for existing labels. Since labels may be used before
they are defined (forward jumps), all labels found are mapped to fresh
symbols. References to the same label (use or definition) will remain
consistent after this transformation (both the use and the definition of
some label l will be mapped to the same label l').
Note: If the tree fragment passed to the duplicator contains unbound
label names, the bind to the outer labeldef will be lost! That's because
a use of an unbound label l will be transformed to l', and the corresponding
label def, being outside the scope of this transformation, will not be updated.
All LabelDefs are entered into the context label map, since it makes no sense
to delay it any more: they will be used at some point.