Fn

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- added example of using tail call optimization with changing callable

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- ``iters.accumulate`` - backported version for Python < 3.3
- first implementation for ``monad.Option`` with tests and README samples

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- ``fn.Stream`` slice is another ``fn.Stream``
- ``fn.Stream`` got new public method ``cursor`` to get position on next evaluated element

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- Update documentation with special ``fn._`` use cases for interactive shells
- Move ``zipwith`` from ``fn.iters`` to ``fn.op``
- ``fn._`` dump to string

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- Added 22 itertools recipes to ``fn.iters``
- Documentation is converted to RST

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- Unit tests coverage for ``fn.stream.Stream``
- ``_StreamIterator`` works fine both in Python 2/3