Remove calls to Object#object_id

[amomchilov]

Ruby 3 added GC compaction, which generally improves performance, but complicates Object#object_id (it's no longer as simple as twiddling with the object's address, since that's no longer stable). Since Ruby 2.7 object_id became more expensive, particularly on its first call for a particular object. There's giant WeakMap under the hood which tracks the association between objects and their IDs. Here's a great blog post about it.

This PR removes usages of object_id, and replaces them with faster alternatives. See the individual commits for details.

Testing with a 5 MB sample PDF, CombinePDF.load(file).save("file.pdf) gets about 1% faster

[boazsegev]

Thanks! 😀🙏🏻

I love the use of compare_by_identity and equal? which I didn't properly dig into before.