IMPORT: slz: import slz into the tree

IMPORT: slz: import slz into the tree

SLZ is rarely packaged by distros and there have been complaints about
the CPU and memory usage of ZLIB, leading to some suggestions to better
address the issue by simply integrating SLZ into the tree (just 3 files).
See discussions below:

   https://www.mail-archive.com/haproxy@formilux.org/msg38037.html
   https://www.mail-archive.com/haproxy@formilux.org/msg40079.html
   https://www.mail-archive.com/haproxy@formilux.org/msg40365.html

This patch does just this, after minor adjustments to these files:
  - tables.h was renamed to slz-tables.h
  - tables.h had the precomputed tables removed since not used here
  - slz.c uses includes <import/slz*> instead of "slz*.h"

The slz commit imported here was b06c172 ("slz: avoid a build warning
with -Wimplicit-fallthrough"). No other change was performed either to
SLZ nor to haproxy at this point so that this operation may be replicated
if needed for a future version.