Remove Hash and Eq from AstNodeRef for types not implementing Eq or Hash

[MichaReiser]

Summary

This is a follow up to #15763 (comment)

It reverts the change to using ptr equality for AstNodeRefs, which in turn removes the Eq, PartialEq, and Hash implementations for AstNodeRefs parametrized with AST nodes.
Cheap comparisons shouldn't be needed because the node field is generally marked as [#tracked] and #[no_eq] and removing the implementations even enforces that those
attributes are set on all AstNodeRef fields (which is good).

The only downside this has is that we technically wouldn't have to mark the Unpack::target as #[tracked] because
the target field is accessed in every query accepting Unpack as an argument.

Overall, enforcing the use of #[tracked] seems like a good trade off, espacially considering that it's very likely that
we'd probably forget to mark the Unpack::target field as tracked if we add a new Unpack query that doesn't access the target.

Test Plan

cargo test

[carljm]

Looks good, modulo the one inline question.

To be clear, my comment wasn't intended to suggest that I think it's important we make this change; I was just curious what the tradeoffs are and if there was anything we should document for future.

[MichaReiser]

Hmm, it does seem that this regresses performance slightly (2%) for the cold case. Let me re-run to verify that it is real. It might just be the cost of tracking the Unpack::target field.