Interleave eagerly calls GetEnumerator

[Orace]

As discussed here in #724, with current implementation of Interleave,
The code below throws:

var sequenceA = TestingSequence.Of(1);
var sequenceB = new BreakingSequence<int>();
var interleave = sequenceA.Interleave(sequenceB).Take(1).ToList();

While we can expect interleave to be a list of one element.