A marker class whose purpose is to add compile-time type safety to node annotations.
Replaces annotation key names. Usage is similar to annotation key names, but annotation key
names are distinguished by equality, while AnnotationKey objects are distinguished by identity.
Thus, this class does not override hashCode and equals.
Note: we could add a name to objects of this class, which would probably be nice for debugging,
but could lead to confusion: one might think that the name is the actual key, but is isn't,
and cannot be, because we cannot associate the name with the type of the annotation - at run-time,
the type is gone because of erasure. Two annotation key objects with the same name would still
be completely unrelated keys.
TODO: as a dedugging aid, call Thread.currentThread.getStackTrace in the constructor and
save class name, method name and line number of the place where the annotation key was created.
Probably the second element in the stack trace array, but that's not guaranteed.