zstd: upgrade vendored zstd to 1.5.6

Files are based on upstream commit
794ea1b0afca0f020f4e57b6732332231fb23c70 / tag v1.5.6.

The unified library has been regenerated and headers have been
synchronized.

The command I used to produce the unified library was
`python3 combine.py -r ../../lib -o zstd.c zstd-in.c`.

Author: indygreg