search graph: lazily update parent goals

[lcnr]

Based on top of #143054. In the search graph only the last entry is actually mutable and all other entries get lazily mutated when popping child goals.

This simplifies a bunch of possible future optimizations:

• We can try evaluating nested goals and entirely ignore discard their evaluation by simply not calling fn update_parent_goal
• Because we only lazily update, tracking the "impact" of a nested goal is easy. The necessary information has to be integrated in the StackEntry of the current goal, as there is otherwise no way to influence its parents. This makes it easier to avoid rerunning cycle heads if they have only been used in candidates which don't impact the final result of a goal.

r? @compiler-errors @BoxyUwU

[rustbot]

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