Remove member_offset_iterator

[tautschnig]

It was undocumented and its fields did't carry descriptive names, thus code using it was hard to read and understand. With all the preceding work it only had a single user left.

Only the first commit is new. Marking do-not-merged until #3167 is merged.

• 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.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[martin-cs]

Shout if / when you want this reviewed.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 6ea2bb7).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89563852
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.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: acb0a05).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89816087