Investigate hierarchical reporting of expansion events in the compiler

[countvajhula]

We use emit-local-step to report compiler transformations as expansion steps, but this API doesn't capture the (tree-structured) context of expansion as normal Racket expansion does. Try unpublished expansion event APIs to see if there are any that already support this. This would help us debug expansion and compilation using the Macro Stepper.

See Context is Everything.