Rework ChunkedItemsIterator

[theofidry]

• Introduce a ::fromStream() static factory which makes it more convenient to create the iterator for STDIN whilst allowing testing easily without relying on STDIN
• Rename ::create() to ::fromItemOrCallable()
• Move the normalization of items from __construct() to the static factories, this makes testing a lot easier
• Simplify some tests: when testing the static factories we are only interested in the items themselves since the batchSize is just forwarded. The chunk logic is tested when testing the constructor directly