Several core iterators introduced

Next steps:

1. Knock out the rest of the Python `itertools` iterators
2. Create a vignette explaining the concepts
3. Create a recipes vignette along the lines of Python's
4. Determine whether we should port any other iterators from `itertools` on CRAN
5. Determine how to more naturally iterate through iterators (e.g., `for (i %in% iter(...))`)