Make Rule's unasync functions public for overwriting

[robsdedude]

I propose to make the unasync functions of Rule public so that custom Rule implementations can more easily inherit from Rule and change the exact replacement mechanisms.

Here are my use-cases that this would enable without having to rely on internal APIs:

• I have code examples in doc strings and I want to unasync those as well. Hence the need for a custom unasync_string)
• I have doc references that I want to unasync. E.g., from :ref:`async-thingamajig` to :ref:`thingamajig`.
• I want to have a more complex replacement customization that just a hard-coded mapping of identifiers. E.g. something like this

  len(name) > 6 and name.startswith("async_"):
      return name[6:]