Deflate

* Slight improvements to decompression speed.

* Added an AVX-512BW implementation of Adler-32.

* The Makefile now supports a user-specified installation `PREFIX`.

* Fixed build error with some Visual Studio versions.

* Fixed crash in CRC-32 code when the prebuilt libdeflate for 32-bit Windows was
called by a program built with Visual Studio.

* Improved the worst-case decompression speed of malicious data.

* Fixed build error when compiling for an ARM processor without hardware
floating point support.

* Improved performance on the PowerPC64 architecture.

* Added soname to `libdeflate.so`, to make packaging easier.

* Added `make install` target to the Makefile.

* The Makefile now supports user-specified `CPPFLAGS`.

* The Windows binary releases now include the import library for
`libdeflate.dll`. `libdeflate.lib` is now the import library, and
`libdeflatestatic.lib` is the static library.

* Added support for multi-member gzip files.

* Moved architecture-specific code into subdirectories. If you aren't using the
provided Makefile to build libdeflate, you now need to compile `lib/*.c` and
`lib/*/*.c` instead of just `lib/*.c`.

* Added an ARM PMULL implementation of CRC-32, which speeds up gzip compression
and decompression on 32-bit and 64-bit ARM processors that have the
Cryptography Extensions.

* Improved detection of CPU features, resulting in accelerated functions being
used in more cases. This includes:

* Detect CPU features on 32-bit x86, not just 64-bit as was done previously.

* Detect CPU features on ARM, both 32 and 64-bit. (Limited to Linux only
currently.)

* Build fixes for certain platforms and compilers.

* libdeflate now produces the same output on all CPU architectures.

* Improved documentation for building libdeflate on Windows.

* Fixed a very rare bug that caused data to be compressed incorrectly. The bug
affected compression levels 7 and below since libdeflate v0.2. Although there
have been no user reports of the bug, and I believe it would have been highly
unlikely to encounter on realistic data, it could occur on data specially
crafted to reproduce it.

* Fixed a compilation error when building with clang 3.7.

* Various improvements to the gzip program's behavior.

* Faster CRC-32 on AVX-capable processors.

* Other minor changes.