module providing write-once, read-many attributes for Moose

 MooseX::SetOnce provides the SetOnce attribute which lets your class have
 attributes that are not lazy and not set, but that cannot be altered once
 set.
 .
 The logic is very simple: if you try to alter the value of an attribute with
 the SetOnce trait, either by accessor or writer, and the attribute has a
 value, it will throw an exception.
 .
 If the attribute has a clearer, you may clear the attribute and set it again.