Auto merge of #151853 - Zalathar:feed-vtable, r=oli-obk

Use the query vtable in `query_feed` plumbing

The `query_feed` function needs to be able to do two important things with (erased) query values: hash them, and debug-print them.

Both of those are things that the query's vtable already knows how to do. So by passing in a vtable to `query_feed`, we can give it a nicer signature, avoid having to unerase values in the function itself, and clean up some caller-side code as well.

Author: bors