Isal

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+ Added an ``igzip_lib`` module which allows more direct access to ISA-L's
igzip_lib API. This allows features such as headerless compression and
decompression, as well as setting the memory levels manually.
+ Added more extensive documentation.

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+ Fix a bug where a AttributeError was triggered when zlib.Z_RLE or
zlib.Z_FIXED were not present.
+ Add support for Linux aarch64 builds.
+ Add support for pypy by adding pypy tests to the CI and setting up wheel
building support.

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+ Fix a bug where multi-member gzip files where read incorrectly due to an
offset error. This was caused by ISA-L's decompressobj having a small
bitbuffer which was not taken properly into account in some circumstances.

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+ Speed up ``igzip.compress`` and ``igzip.decompress`` by improving the
implementation.
+ Make sure compiler arguments are passed to ISA-L compilation step. Previously
ISA-L was compiled without optimisation steps, causing the statically linked
library to be significantly slower.
+ A unused constant from the ``isal_zlib`` library was removed:
``ISAL_DEFAULT_HIST_BITS``.
+ Refactor isal_zlib.pyx to work almost the same as zlibmodule.c. This has made
the code look cleaner and has reduced some overhead.

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+ Remove workarounds in the ``igzip`` module for the ``unconsumed_tail``
and ``unused_data`` bugs. ``igzip._IGzipReader`` now functions the same
as ``gzip._GzipReader`` with only a few calls replaced with ``isal_zlib``
calls for speed.
+ Correctly implement ``unused_data`` and ``unconsumed_tail`` on
``isal_zlib.Decompress`` objects.
It works the same as in CPython's zlib now.
+ Correctly implement flush implementation on ``isal_zlib.Compress`` and
``isal_zlib.Decompress`` objects.
It works the same as in CPython's zlib now.

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+ Fix a crash that occurs when opening a file that did not end in ``.gz`` while
outputting to stdout using ``python -m isal.igzip``.