fix: dynamically update deep scopedSlot refs

[chearon]

#764 works perfectly when you're using normal slots, but not if you forward scoped slots to a child component.

Unfortunately, and I hope I'm mistaken about this, every component's entire VNode tree has to be iterated to find a component that needs refs updated.

Here's some perf testing of this PR in our application. In the Firefox profiler, afterRender shows up for:

• 0.5ms in a 1,000-node component
• 15ms in a 50,000-node component

Ideally this would use Vue's patch hooks like native Vue 2 refs do. Would it be out of the question to introduce a faster v-ref= instead of this PR? Or is this fast enough given that the readme already warns about performance issues?