[WIP] Dictionary compressed arrangements

[frankmcsherry]

Evolving PR for enabling dictionary compression in arrangements.

Roughly, as each row: &[u8] is presented, we'll rip it apart into columns, and look for the option to use otherwise unused byte patterns (tags, otherwise used for row decoding) to reference popular values in each column. Columns are encoded independently, so things can be differently popular in different columns.

Fair bit of work still to do, but checking in for the moment.

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.