Implements a hash or dictionary. This object holds name/object pairs. Objects may be added and removed.

Factory Messages
create Creates an empty hash
size: size Creates a hash with a particular minimum size

Operational Messages
at: name put: oid P2Object add an object to the hash
at: name P2Object retrieves an object by name
remove: name P2Object removes the named object from the hash
size int get the number of objects in the hash