transformers should log their changes

[telic]

Some transformers (especially the DeprecationTransformer) can make a large number of changes to a DOM which are largely opaque from the user's perspective. These transformers would be a lot less scary to use if they logged a message for every modification they made so that the user could review said changes after the fact.

A textual diff gets us half way there, but log messages would be able to offer explainations for //why// each change took place, and show how multiple textual changes are related (eg. moving something from one place to another).