Restore read barrier infrastructure

[wycats]

Before the recent internals overhaul, we had infrastructure that allowed renderers to register a handler that should be called when a reactive value was consumed outside of a tracking frame.

For example, the React renderer uses this infrastructure to warn the user that useReactive is necessary, but only when the consumption in question happens while a React render function is running.

The Vue renderer needs similar infrastructure for a similar reason.

The internals are currently structured to support the read barrier feature; it just needs a more stable public API and implementation.