Load as a digested dictionary to compressor.
Pass this attribute as zstd_dict argument:
compress(dat, zstd_dict=zd.as_digested_dict)
1. Some advanced compression parameters of compressor may be
overridden by parameters of digested dictionary.
2. ZstdDict has a digested dictionaries cache for each compression
level. It's faster when loading again a digested dictionary with
the same compression level.
3. No need to use this for decompression.