Bitsets

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Use private ``_int`` attribute for internal unboxing purposes.

Relax ``bitsets`` and ``graphivz`` dependencies to < 1.0.

Improved documentation.

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Raise an error if ``bitset`` members have duplicates.

Extended and improved documentation, added Sphinx-based API reference.

Improved unittests.

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Added ``integers.n()`` and ``integers.rank()``.

Make ``.iter_set()`` available on series as ``.index_sets()`` method.

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Made ``.indexes()`` available on bitsets as ``.iter_set()`` method.

Added ``transform`` module providing additional integer (un)packing tools.

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Added optional ``as_set`` parameter to ``.members()`` method returning a ``frozenset``.

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Added optional boolean value argument to ``.count()`` method.

Added ``integers.bit_mask()``.