Secondarily:
- If we take this step, is `IdentityObject` still the best name?
Possible candidates:
- Identity, as in "class Foo implements Identity". This says what it
means, though it is quite possible that it will clash with existing
types. (This is not an absolute disqualifier, but it is a consideration.)
- IdentityObject. This is where we are now; it always felt a little
clunky to me.
- ObjectIdentity ("class Foo implements ObjectIdentity"). Better than
IdentityObject, less likely to clash than Identity.
- WithIdentity. Not the best name, but less likely than Identity to
clash.
Others?
