[aliassar]

mutmut apply looked up the function to replace by its bare name and took the first top-level function or method with that name, so applying a mutant of a top-level size could overwrite Box.size when the class came first in the module. The class name is already in the mutant key, so apply_mutant now passes it to a new find_function helper that searches that class's methods, or only top-level functions when the key has no class. It's a separate helper because read_original_function and read_mutant_function look up the generated names, which already include the class, so mutmut show and the browse diff were never affected. It follows the snippet Ivan Balashov suggested in the issue, thanks for the clear report. The new test applies a mutant of a top-level function and of the methods of two classes that share the name size, with and without the line span index, and checks that only the right function changes. I also ran mutmut run, mutmut apply and mutmut show on the repro from the issue for both mutant keys.

Fixes #624