[Rename refactoring] Stricter checks & tests for change annotations

[toinehartman]

Problem

Rename refactoring supports annotating suggested edits with a label/description, and a 'needs confirmation' flag. Currently, due to programmer error, if an edit to the same location is registered multiple times, annotations could overwrite each other, leading to unpredictable behaviour. Currently, annotations are only use for instances of has, but when they become more widespread, this can lead to unexpected results.

Requested solution

1. Impose stricter checks on change annotations in the unit test framework.
2. Refactor rename location<>change annotation bookkeeping to make this less likely.