Move flatten_byte_* to lowering [blocks: #2068, #2501]

[tautschnig]

The operations in this code are not tied to Boolean flattening and instead do rewrites that are usable within any back-end.

This is a first step towards getting #2068 in a shape that can be reviewed and merged. No functional changes in this PR.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 57f8848).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90582269
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[peterschrammel]

If flattening is different from lowering then why are the functions in lowering called flattening?

[tautschnig]

They will be renamed (I had already done so in 8cd566e), but I wanted to keep this one as easy as possible to review. I'll follow up with a PR doing the renaming shortly.