This PR implements Enumerator#feed and addresses #1571. However the behaviour is not same as MRI because of the difference in implementation of next in JRuby and MRI.

Here is an example to demonstrate the difference,

MRI

a = [1, 2, 3]
m = a.to_enum(:map!)

m.next # a -> [1, 2, 3], returns 1
m.next # a -> [nil, 2, 3], returns 2
m.next # a -> [nil, nil, 3], returns 3
m.next # a -> [nil, nil, nil],  throws StopIteration exception and calls the yield/method

JRuby

a = [1, 2, 3]
m = a.to_enum(:map!)

m.next # a -> [nil, 2, 3], returns 1
m.next # a -> [nil, nil, 3], returns 2
m.next # a -> [nil, nil, nil], returns 3
m.next # a -> [nil, nil, nil],  throws StopIteration exception and doesn't call yield/method

So basically when you call next first time in MRI, it actually initialises the block but doesn't call it yet unlike JRuby.

So because of this I haven't included test_feed specs in TestEnumerable (test:mri), as they fail because of the different order of evaluation of yield/method.

Though this PR does implement feed correctly for JRuby's next implementation, so I am raising it.