pu/zstd

Introduce support for zstd.

Measurements

Archive size: We recompressed Ubuntu main amd64 and it is now 5.9 GB instead of 5.6 GB. Our main use case is installing packages with eatmydata (for setting up cloud instances), and we see about 30-40% performance increase there (firefox on a basic debootstrap schroot):

before:

8.79user 2.87system 0:12.43elapsed 93%CPU (0avgtext+0avgdata 25416maxresident)k
0inputs+548016outputs (0major+384193minor)pagefaults 0swaps

after:

4.24user 2.57system 0:08.54elapsed 79%CPU (0avgtext+0avgdata 25280maxresident)k
0inputs+544584outputs (0major+392117minor)pagefaults 0swaps

Memory usage for xz -6 and zstd -19 is comparable.