Support for compression/decompression with dictionary

[coolbutuseless]

Support zstd dictionaries.

• Load a file as a dictionary.

  • The dictionary needs to be initiated into a specific C struct. This takes a "non-negligible" amount of time according to docs.
  • So initialize structure and cache in external pointer.

• Create some generic dictionaries to include with the package

  • train on (a) generic vectors of integer, numeric, character vectors
  • train on (b) a specific dictionary for small integer vectors, small numeric vectors, small character vectors, small factors etc

[coolbutuseless]

dictionaries supported when creating contexts with init_zstd_cctx() and init_zstd_dctx()

https://github.com/coolbutuseless/zstdlite/commit/6f57be54b53848cfa6b7883d1d0719cb2b06a30f