add new port archivers/zstd

Zstd, short for Zstandard, is a real-time compression algorithm providing
high compression ratios. It offers a very wide range of compression vs.
speed trade-offs while being backed by a very fast decoder. It offers
a special mode for small data called "dictionary compression" and it can
create dictionaries from any sample set. Zstd is BSD-licensed.

Using Izbench on the Silesia compression corpus, zstd ranked at the
top with a compression ratio of 2.877, a compression rate of 325 Mb/s,
and a decompression rate of 325. Zlib followed at 2.730, 95 Mb/s (C)
and 360 Mb/s (D). See WWW page for the full benchmark results.