class ZAML::Label

YAML only wants objects in the datastream once; if the same object

occurs more than once, we need to emit a label ("&idxxx") on the
first occurrence and then emit a back reference (*idxxx") on any
subsequent occurrence(s).

To accomplish this we keeps a hash (by object id) of the labels of

the things we serialize as we begin to serialize them.  The labels
initially serialize as an empty string (since most objects are only
going to be be encountered once), but can be changed to a valid
(by assigning it a number) the first time it is subsequently used,
if it ever is.  Note that we need to do the label setup BEFORE we
start to serialize the object so that circular structures (in
which we will encounter a reference to the object as we serialize
it can be handled).