class Executor

An Executor encapsulates the kernel state necessary to implement the Zircon system calls. It

depends on an interface from the kernel below it, presenting primitives like threads and wait

queues. It presents an interface to the system call implementations.

The goals of factoring this into such a layer include:

- The ability to test code in this layer separately from low-level kernel implementation details,

and from the syscall mechanism. This includes correctness as well as performance tests.

- Centralize resource management in order to make progress on things like not reporting

ZX_ERR_NO_MEMORY when creating a zx::event, or reporting bad handle faults.

TODO(kulakowski) The above comment is aspirational. So far, only the root job (and its observer)

is managed by the Executor. Other subsystems, like port arenas and handle arenas, are not yet

included. And e.g. tests are not yet written against the Executor.