Faster dedup instance graph (#1732)

* dedup: add faster InstanceGraph implementation and use it in dedup

The new implementation takes care not to hash the instance
types contained in DefInstance nodes.
This should make dedup considerably faster.

* FastInstanceGraph: cache vertices for faster findInstancesInHierarchy

* FastInstanceGraph: remove the parent name field since it isn't actually necessary

* FastInstanceGraph -> InstanceKeyGraph

* InstanceGraph: describe performance problems.

* InstanceKeyGraph: turn moduleMap into a def instead of a val

This will make changing implementation details much easier
in the future.

* InstanceKeyGraph: return childInstances as Seq instead of Map

This ensures a deterministic iteration order and it
can easily be turned into a Map for O(1) accesses.

* InstanceKeyGraph: add tests for public methods

* InstanceKeyGraph: group public methods together

* InstanceKeyGraphSpec: fix wording of a comment

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

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