Changes from 1.18.1 to 1.19.0

The length of automatic blocksizes for fast codecs (lz4, blosclz) has
been incremented quite a bit (up to 256 KB) for better compression ratios.
The performance in modern CPUs (with at least 256 KB in L2 cache) should
be better too (for older CPUs the performance should stay roughly the same).
Continuous integration has been migrated to GitHub actions and much
more scenarios are tested (specially linking with external codecs).
Also, a new OSS-Fuzz workflow has been added for increased detection
of possible vulnerabilities. Thanks to Nathan Moinvaziri.
For small buffers that cannot be compressed (typically < 128 bytes),
blosc_compress() returns now a 0 (cannot compress) instead of a negative
number (internal error). See #294.
Thanks to @kalvdans for providing the initial patch.
blosclz codec updated to 2.1.0. Expect better compression ratios and
performance in a wider variety of scenarios.
blosc_decompress_unsafe(), blosc_decompress_ctx_unsafe() and
blosc_getitem_unsafe() have been removed because they are dangerous
and after latest improvements, they should not be used in production.
zstd codec updated to 1.4.5.
Conan packaging has been deprecated (from now on, we should try
to focus on supporting wheels only).

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