Stores config diffs for display on test failure.

Used internally by Mace.cleanup/1 in on_exit callbacks. The diff data is read by an ExUnit formatter (registered separately) that injects diffs into test failure output.

Usage in tests

setup context do
  Mace.set(:my_app, :timeout, 100)
  on_exit(fn -> Mace.cleanup(context) end)
  :ok
end

The cleanup/1 call records the current process's config diffs (keyed by the test's module and name) and then resets the config. On test failure, the formatter reads and displays the diff.