Deflate

* The CRC-32 checksum algorithm has been optimized with carryless multiplication
instructions for `x86_64` (PCLMUL). This speeds up gzip compression and
decompression.

* Build fixes for certain platforms and compilers.

* Added more test programs and scripts.

* libdeflate is now entirely MIT-licensed.

* The Adler-32 checksum algorithm has been optimized with vector instructions
for `x86_64` (SSE2 and AVX2) and ARM (NEON). This speeds up zlib compression
and decompression.

* To avoid naming collisions, functions and definitions in libdeflate's API have
been renamed to be prefixed with `libdeflate_` or `LIBDEFLATE_`. Programs
using the old API will need to be updated.

* Various bug fixes and other improvements.

* Some bug fixes and other minor changes.

* Implemented a new block splitting algorithm which typically improves the
compression ratio slightly at all compression levels.

* The compressor now outputs each block using the cheapest type (dynamic
Huffman, static Huffman, or uncompressed).

* The gzip program has received an overhaul and now behaves more like the
standard version.

* Build system updates, including: some build options were changed and some
build options were removed, and the default 'make' target now includes the
gzip program as well as the library.

* Initial official release.