Explode zstd 1.4.5 into separate upstream files
It's much nicer to import from upstream this way, and compiles
faster too.
Everything in lib/ is unmodified 1.4.5.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12978