Various internal MultiIndex improvements

[vyasr]

This PR is a follow-up to #9191 to further simplify the MultiIndex class. It removes various unused functions, inlines and simplifies a number of other single-use internal functions, and moves more methods to Frame that can be shared. It also makes numerous other miscellaneous improvements to the code in MultiIndex to simplify further rewrites.

[vyasr]

Two notes:

• I've marked this as breaking because the labels parameter was removed from the constructor. This parameter has been deprecated for over two years and should be safe to remove at this point.
• I've targeted this at 21.10 to take advantage of CI for the moment but we can safely retarget to 21.12 if we don't want to squeeze this in at this point. There's nothing critical in this PR.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.12@079af45). Click here to learn what that means.
The diff coverage is n/a.

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  Hits                    ?     2080           
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[thomcom]

A delightful stroll down memory lane. Great job digging through the miserable special cases of MultiIndex, thanks!

[vyasr]

Temporarily leaving this PR pointing to branch-21.10 to for CI, but moving to the 21.12 milestone. Will retarget the PR once changes are finalized (or 21.12 CI is up and running).

[marlenezw]

Overall this is looking good to me. It might be able to make it into 21.10 if there aren't any other major changes (not sure when burn down is though.)

[vyasr]

@galipremsagar once you're happy with this it's up to you whether you want to merge this into 21.10 or 21.12. It seems like it's basically ready, but as we discussed offline (and commented on this thread above) it may be wiser to hold off until 21.12 to avoid any last minute surprises.

[galipremsagar]

LGTM, lets merge this into branch-21.12 as discussed offline. Can you rebase the changes?

[vyasr]

Wow I completely borked the merge somehow. Going to rebase, will need to remove unnecessary requested reviewers.

[vyasr]

Ah I think I see the problem. Has forward-merge from 21.10->21.12 not been set up yet? I think I had commits from merging 21.10 in my PR that were not yet present on 21.12.

[vyasr]

I had fixed a typo in a test created in #9258, which led to conflicts from merges. I've removed that change entirely to avoid causing issues with the 21.10->21.12 forward merge once it's introduced.

[vyasr]

@gpucibot merge