Chunk function to return an Iterator of array chunks

 This function yields arrays of the specified size

 Examples:

      iter\chunk([1, 2, 3], 2)
      => iter([1, 2], [3])
Reorder functions in README

Group into functions transforming iterators, those creating new
ones and those reducing them to other types.
Merge remote branch 'camspiers/chunk'

Conflicts:
	README.md
	src/iter.rewindable.php
	test/iterTest.php
More chunk tests