Add a Node#hierarchy method for flexible accounting.

[havenwood]

This PR adds a Node#hierarchy for more flexibility with outputting hierarchy. I switched over Node#print_hierarchy to use the new method internally.

My hope was to make Node hierarchy discoverable via a mechanism other than printing.

For an example, here's a typical #print_hierarchy output.

#<Async::Node:0x000118>
	#<Async::Node:0x000078>
		#<Async::Node:0x0000a0>
			#<Async::Node:0x0000c8>
	#<Async::Node:0x0000f0>

And the corresponding current #hierarchy return value.

[{:level=>0, :node=>#<Async::Node:0x000118>},
 {:level=>1, :node=>#<Async::Node:0x000078>},
 {:level=>2, :node=>#<Async::Node:0x0000a0>},
 {:level=>3, :node=>#<Async::Node:0x0000c8>},
 {:level=>1, :node=>#<Async::Node:0x0000f0>}]

[coveralls]

Coverage increased (+3.8%) to 89.574% when pulling d1f534e on havenwood:hierarchy into ce8bc07 on socketry:master.

[ioquatix]

Do you mind explaining the expected use case? Because we have traverse which is very close to the implementation.

[havenwood]

Do you mind explaining the expected use case? Because we have traverse which is very close to the implementation.

If I recall, I was using #traverse to return a collection of nodes, like .traverse.map { |node, value| would but got a LocalJumpError. In retrospect, I think I should have proposed an return enum_for(__method__) { size } unless block_given? form for #traverse rather than a new method.