Updated the lz4 decompressor

As an experiment, I stole the lz4 decompressor from
upstream lz4 (1.9.3), and landed it.

Feedback suggested that keeping the vendor lz4 code isolated and
unlinted was probably reasonable, so I lobbed it into its own file.

It also seemed reasonable to put the mostly-untouched* code into
lz4.c proper, and relegate the integrated and ZFS-specific code to
lz4_zfs.c.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12805