modules/zstd: Add sequence decoding

[rw1nkler]

This PR is meant to provide a proc implementing sequence section decoding as described in: https://datatracker.ietf.org/doc/html/rfc8878#section-3.1.1.3.2

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

[proppy]

is it independent from #1315 or should they be reviewed together?

[rw1nkler]

is it independent from #1315 or should they be reviewed together?

We agreed that this should be reviewed as an independent PR after merging #1315

[proppy]

should this be rebased now that #1315 has been merged?

[rw1nkler]

Superseded by https://github.com/google/xls/pull/1857