Weave as a background service

[mratsim]

This is an experimental implementation of #132

Direct link to the readme: https://github.com/mratsim/weave/tree/8f9db32b71cb1e493c496d8afce44aba38f9d6c6#foreign-thread--background-service-experimental

Weave can now be started in a dedicated thread via runInBackground(Weave).

Jobs can be submitted to it and waited upon with submit fnCall(args) and waitFor(Pending).
To do that, a thread (unmanaged by Weave) must call setupSubmitterThread(Weave) and then waitUntilReady(Weave).

A job is different from a task in only small ways.

• a job is called from a thread that Weave doesn't know about
• jobs are processed in FIFO order to minimize latency while tasks are in LIFO order to maximize locality, cache reuse and throughput
• jobs can spawn tasks (and tasks can spawn tasks). Concretely that means that
  when a job enters the runtime, work on a new job will wait for all subtasks to be processed unless the job does not spawn enough tasks to create work for all CPUs.

To ensure proper usage of submit/waitFor on submitter threads and spawn/sync within Weave tasks a distinct Pending type has been created with the same semantics as Flowvar.

In the future, submit/waitFor may be removed for an unified spawn/sync construct.