Replace lrzip with zstd

[AlexDaniel]

See #23 (comment) for some context. When lrzip was introduced, zstd did not have a long-range mode, so lrzip was the only choice to achieve what we needed. Today the situation is different, and we can use zstd for everything.

[AlexDaniel]

Actually, because of facebook/zstd#3062, it's not a win-win situation.

For 20 rakudo builds the results are like this:

Command	Compression time	File size	Decompression time
lrzip -q -L 9	≈26s	≈8M	≈5.3s
zstd -19 --long=31	≈97s	≈12M	≈0.8s
zstd --long=31	≈3.7s	≈16M	≈0.8s

It's amazing what zstd can do at just double the size. However, CPU cycles are cheap, so I don't mind if the server uses the idle time to achieve a slightly higher compression rate. That said, I wish zstd was able to reach lrzip's compression ratio.