LazyTransformer fails when multiple values produced per step

[dphilipson]

Running

toIter([[1, 2], [3]], cat)

produces the error

TypeError: Cannot read property 'items' of undefined
    at Object.stepper.@@transducer/step (./node_modules/transducers.js/transducers.js:868:5)
    at Object.newxform.@@transducer/step (./node_modules/transducers.js/transducers.js:743:41)
    at reduce (./node_modules/transducers.js/transducers.js:149:42)
    at Cat.@@transducer/step (./node_modules/transducers.js/transducers.js:747:10)
    at Stepper.@@transducer/step (./node_modules/transducers.js/transducers.js:888:36)
    at LazyTransformer.@@transducer/step (./node_modules/transducers.js/transducers.js:918:38)
    at LazyTransformer.next (./node_modules/transducers.js/transducers.js:903:28)
    at reduce (./node_modules/transducers.js/transducers.js:160:20)
    at toArray (./node_modules/transducers.js/transducers.js:802:12)

where we would expect to get an iterable for the values [1, 2, 3]. For comparison,

toArray([[1, 2], [3]], cat)

produces [1, 2, 3] as expected.

In general, this occurs any time the transducer provided to the constructor of LazyTransformer causes @@transducers/step to be called more than once in its input transformer for a single call in its output transformer. With the transducers provided in this library, this is possible when using the cat and mapcat transducers.