Currently, when a finalizer callback is registered, it is not
guaranteed to be called if there is a global reference to the
corresponding object that survives the isolate. This is because the
finalizer callback takes a `&mut Isolate`, and so it must be called
before the isolate is fully destroyed, but all existing globals
(including possibly the one being currently finalized) are still
usable while there still exists a mutable reference to the isolate.
However, there are still use cases for having finalizers that are
guaranteed to run regardless of any remaining globals, but that don't
require any interaction with the isolate. This change adds them.
This change also changes the context annex to use a guaranteed
finalizer, fixing a bug with context slots not being freed if there
were any globals to the context at the time the isolate is dropped.
Isolate::run_microtasks() already exists but the microtasks queue
is also flushed on script entry and exit. Some embedders want strict
control over when microtasks run, which is set by the policy.
And deprecate Isolate::run_microtasks(). The corresponding V8 API is
tagged with V8_DEPRECATE_SOON.
Local handles never need to be mutable. This patch also rounds up the
last few places where we were still asking the user to pass an `&mut T`
to an API method.
