Allow nested reactors/tasks, improves comparability of APIs about async HOT 5 CLOSED

Hierarchical selectors should be supportable with nio4r (both libev + Java NIO backends) but not necessarily tested yet

from async.

That's interesting - how would that work?

The problem right now is implementing a function like this:

def run_server
    Reactor.run do
        TCPServer...
    end
end

Reactor.run
    #... nested function calls
        run_server
end

The idea being that you don't need to know how run_server is implemented, but within an existing reactor it's asynchronous. The benefits of this are composability - if you simply call run_server, it will work and block while it's working - use it from your top level script, or from a function, or whever, and it just works. But, if you drop it into a run-loop, it works async and doesn't block anything else.

from async.

You can add one selector to another as if it were any other selectable type. Hierarchical selectors are supported natively by various OS APIs such as epoll and kqueue.

from async.

That's awesome - but in order for that to work, we'd also need to support timers?

But I think it's an interesting idea to support in nio4r. Right now, it's not needed to achieve what I want here, I think I can just use the existing reactor if it's available.

from async.

Tasks are now nested using a tree structure.

from async.