zipper: short-circuit on shared subtries

[marcin-rzeznicki]

Enhance merge logic by detecting structural sharing:

• on function entry
• immediately after descending on a shared edge

If all active zippers share the same node, delegate to MergePolicy::on_id
and skip the entire subtree.

[Adam-Vandervorst]

Since this code is contingent on a criterion that does occur in PathMaps loaded from a set (without merkleize), I'd like to see some tests that specifically exercise this codepath.

[marcin-rzeznicki]

Since this code is contingent on a criterion that does occur in PathMaps loaded from a set (without merkleize), I'd like to see some tests that specifically exercise this codepath.

Fixed in 9ff3982 - I think the test gives us enough coverage

[Adam-Vandervorst]

Yes, at least this is some sniff test, and we can ramp up the testing with the property-based testing when we move this out of experimental.

[Adam-Vandervorst]

I'm good with merging it now.

[marcin-rzeznicki]

Thanks! @luketpeterson do you have any remarks?