An identifier class for REEF. Identifiers are a generic naming primitive
that carry some information about the type of the object they point to.
Typical examples are server sockets, filenames, and requests.
Identifier constructors should take zero arguments, or take a single string.
Return a string representation of this object. This method should return a
URL-style string, that begins with "type://", where "type" is chosen to
uniquely identify this type of identifier.