Followup work from scala/scala#6093

We couldn't do this in 2.12 due to the source incompatibility, but we might be willing to break this for 2.13.

If we do, we, need to:

• factor the code in refchecks into something we can share with FastTrack's use of scala.tools.reflect.FormatInterpolator.
• after the subsequent restarr, replace StringContext.{s,f} with fast tracked macros.

The benefit of a macro over an intrinsic is that we avoid divergence between the intrinsified and regular behaviour. Currently, the intrinsic defers the the regular behaviour when the tree shapes are non-standard, e.g. a manual call to StringContext.s with non-literal arguments for parts. The f interpolator, which is already a macro, just emits a compile error for that case.