Ask politely for a stable macro expander hook

At present, resyntax relies on the undocumented and deeply magic current-expand-observe parameter (intended for the macro stepper) to apply refactoring rules to visited syntax objects. This has a few problems:

• It's an unstable API whose details change whenever the macro expander or macro stepper changes.
• It provides way more information than resyntax needs. All resyntax needs is a list of original syntax objects visited during expansion.
• It doesn't provide a way to pause expansion, so there doesn't seem to be a way for refactoring rules to use local-expand or syntax-local-value.

Eventually it would be nice to have a public, documented, stable, and less powerful alternative to current-expand-observe in Racket.

Start by reading how resyntax currently relies on the undocumented current-expand-observe parameter. The issue calls for a public, documented, stable, less powerful alternative that provides visited syntax objects and can pause expansion for local-expand or syntax-local-value. Done means the alternative exists and addresses these stated limitations.