Tighten the backtrace pollution from passing through callbacks

Callbacks are everywhere, so it's better if we can avoid making a mess
of the backtrace just because we've passed through a callback hook.

I'm making no effort to the before/after invocations: those only affect
backtraces while they're running. The calls that matter are the ones
that remain on the call stack after run_callbacks yields: around
callbacks, and internal book-keeping around the before/afters.