Speed up equals method on ImmutableOpenMap

[original-brownbear]

This method is somewhat hot during CS diffing at times, we can speed it up a little by taking the pointless indirection of the conversion iterator out of things and saving the allocations from it.

relates #77466

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[thecoop]

What sort of perf bump do we get from this?

[original-brownbear]

What sort of perf bump do we get from this?

Hard to put a clean measurement on this. Not a massive amount in terms of runtime but two nice things come out of this:

1. This saves ~1% of all object allocations in the many-shards snapshot benchmark which isn't nothing and saves CPU indirectly
2. This seems to make map equals on other maps faster in some diff spots by apparently removing some metamorphic call sites in AbstractMap

I mostly care about the first point, taking a little of the GC noise out of things.

[thecoop]

Fair enough, LGTM

[original-brownbear]

Thanks Simon!