modules/zstd: Add literals decoding

[rw1nkler]

This PR is meant to provide a proc implementing literals section decoding as described in: https://datatracker.ietf.org/doc/html/rfc8878#section-3.1.1.3.1

In the beginning, we will focus on implementing the entire RAW and RLE literals decoding path. Support for literals that use Huffman coding will be added later.

NOTE: this is based on https://github.com/google/xls/pull/1214, please ignore commits from that branch when reviewing.