Change instance variable numbers for line numbers instead of counter

Nobody has claimed this yet.

Whenever we are creating a new instance of a variable or a fresh one, we keep the variable name and concatenate it with an incremental counter (e.g., a0, x1, y2).
A simple change would be to use the line number where this value was introduced, instead of the counter.
Example:

The error could be `#x_25 == #x_24 - 10` where `24` and `25` are the lines where these were introduced.
A question could be, if multiple vars are changed could we still use the same tactic? Lets try and make some tests to see.

Start by locating the code that creates fresh instance-variable names and the existing tests for generated names. Check how line numbers are available at each introduction, then add tests covering the proposed line-based names and cases where multiple variables change together; done means the tests define and confirm the intended naming behavior.

Written by the indexing model from the issue text.