gh-142889: Improve layout of dictionary keys

[brijkapadia]

This is based on the work of @clintonsteiner in PR #145097. Most of it is very similar although I did make my own changes.

This PR updates the memory layout of _dictkeysobject to improve performance.

• Issue: Improved performance and arguably simpler code for dictionaries by changing the keys layout #142889

[markshannon]

Have you asked @clintonsteiner if it is OK for you to take over their work?

[brijkapadia]

@markshannon No, I didn't. I thought it would be okay to create a new PR since the head repo of #145097 was deleted a month ago. However, I can close this if you think this is unfair.

@clintonsteiner Do you want to continue this?

[brijkapadia]

@markshannon, a reminder about this pr, whenever you get a chance.

[markshannon]

I guess we aren't going to hear from @clintonsteiner.

@brijkapadia is this all your own work, or did you use @clintonsteiner's code?
If it is your work, I will be happy to merge it.

Do you have any performance numbers for this?

[brijkapadia]

This is mainly my work.

While there were many similarities to begin with, after gh-150490, I had to restructure the code.

Right now I'm away from my computer, but I could try to get performance numbers next week.