Installing a callback now returns a CallbackHandle. All map-style APIs
install this handle automatically on the created dynamic, which keeps
the callback installed until the dynamic is freed. All other APIs
return the handle for the caller to either call persist() or store
somewhere.
Now, the dynamic system can be used for application-long data with
almost no fear of leaking data due to how callbacks are being installed.
Technically cycles are still possible by moving clones into the
callbacks, so a WeakDynamic type might be worth exposing.
