Blosc2

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* Removed private include in `b2nd.h`. This fixes issue 579.

* Added a new BLOSC_FILTER_INT_TRUNC filter for truncating integers to a
given number of bits. This is useful for compressing integers that are
not using all the bits of the type. See PR 577.

* Optimized zstd, specially when using dicts. See PR 578.

* Initialize grok library when loading the plugin. This is needed for other plugins
to be able to use it without the need of importing the package.

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* New `blosc2_get_slice_nchunks` function for getting the unidimensional chunk indexes of a Blosc2 container slice.

* Globally registered new codec `grok`. This will be loaded dynamically.

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* Frames accept now typesizes that are larger than 255 (and up to 2**31). See https://github.com/PyTables/PyTables/issues/1086. Thanks to chris-allan for the report.

* AVX512 runtime dispatching has been fixed (basically disabled) for GCC versions <= 10.

* Use typedef for blosc_timestamp_t. Thanks to Magnus Ulimoen.

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* Added support for ARMv7l platforms (Raspberry Pi). The NEON version
of the bitshuffle filter was not compiling there, and besides it offered
no performance advantage over the generic bitshuffle version (it is 2x to
3x slower actually). So bitshuffle-neon.c has been disabled by default in
all ARM platforms.

* Also, unaligned access has been disabled in all ARM non-64bits platforms.
It turned out that, at least the armv7l CPU in Raspberry Pi 4, had issues
because `__ARM_FEATURE_UNALIGNED` C macro was asserted in the compiler
(both gcc and clang), but it actually made binaries to raise a "Bus error".

* Thanks to Ben Nuttall for providing a Raspberry Pi for tracking down these
issues.

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* Fix ALTIVEC header. Only affects to IBM POWER builds. Thanks to
Michael Kuhn for providing a patch.