add a batch optimization for the global completer

This change introduces new scheduleFromAnywhere methods which take
a range of Fibers to schedule.

Blockable gets a new method returning the fiber used to start
the unblocked context, which is used by Future/PartialCompletableFuture
to provide a way of completion and returning the continuation Fiber
to the caller so they may schedule the continuation how they want.

If the meson option io_batch_anywhere_completions is set the global
completer will collect all callback and continuation fibers before
scheduling them at once when it is done reaping the completions.
The idea is that taking the AnywhereQueue write lock and calling onNewWork
must only be done once.

TODO: investigate if onNewWork should be extended by an amountOfWork
argument which determines how many worker can be awoken and have work to
do. This should be trivially since our WorkerWakeupSemaphore implementations
already support notify_many(), which may be implemented in terms of
notify_all though.