- Mostly for convenience; it's common to cancel timers even if
  you don't know they are running. Without this, such code would
  need a lot of (fragile) checks.

- Not enough to just redefine the relink() method, need to
  also use it...

- No intention of supporting non-hierarchical wheels, so that part
  of the name was just noise.

- Allows using multiple different callback signatures in the same
  wheel.
- Unfortunately it means having to eat the vtable overhead.
- Add a MemberTimerEvent subclass which is initialized with a
  (dynamic) instance pointer and a (static, templated) member
  function pointer, instead of just a generic Callable.