IMPORT: slz: update slz to version 1.4-dev3

IMPORT: slz: update slz to version 1.4-dev3

This refreshes the slz library to 1.4-dev3, bringing a number of
improvements:
  - x1.4 to x20 compression speed depending on input contents, due
    to better handling of non-compressible data (x20, think CSS/HTML
    with embedded images) and many other optimizations giving at
    least x1.4 at the base.

  - slightly higher default compressed ratio (46.34% vs 46.97%) and
    top compressed ratio (43.03% vs 45.33%) on silesia corpus, and
    23.36% vs 26.22% on linux kernel, which now compresses at 1 GB/s
    vs 600 MB/s before on a P-core of a Core Ultra 265k.

  - x2 to x3 decompression speed (mostly 2.5) depending on input
    stream, best gains on gzip input which was previously much slower
    since not as much in the target.

  - better support for crc32 on ARM (properly ignore older archs)

  - runtime detection of CPU support for PCLMULQDQ on x86_64

This version now passes all lzbench tests so it was the right moment
to update it. More updates to come soon (taking content-type into
account to adjust optimizations) still being worked on, but nothing
critical.

In order to ease updates, just like for xz or ceb, stubs were added
in src/ that directly load the original files from include/import.