Run both local end multi-threaded executors

[oblique]

With this someone can run both Executor and LocalExecutor. Here's an example:

let ex = Executor::new();
let local_ex = LocalExecutor::new();

local_ex.enter(|| ex.run(async { }));

// or

ex.enter(|| local_ex.run(async { }));

[oblique]

Is this acceptable?
Another way is refactoring multitask and introducing LocalTicker.

[Keruspe]

See also #3 for a much simpler (imo) alternative which works fine to do just that with each thread doing some ex.enter(|| local_ex.run(fut))

[ghost]

Closing for the same reason as #6