Change MultiIndex repr (labels -> codes)

[topper-123]

As I mentioned in #22511 (comment), the repr for MultiIndex ATM uses labels, even though the parameter/attribute has been changed to codes.

>>> mi = pd.MultiIndex.from_product([[8,9,0], ['a', 'b', 'c']])
>>> MultiIndex(levels=[[0, 8, 9], ['a', 'b', 'c']],
...            labels=[[1, 1, 1, 2, 2, 2, 0, 0, 0], [0, 1, 2, 0, 1, 2, 0, 1, 2]])

This PR changes the repr, so it uses codes.

Fot the record I think we should either decide to adapt #22511 soon, or soon come up with an consensus about an alternative repr to be implemented. The current MultiIndex repr is really, really (really!) bad for large indexes and needs to be be changed.

[codecov]

Codecov Report

Merging #24731 into master will decrease coverage by 49.32%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #24731       +/-   ##
===========================================
- Coverage   92.39%   43.07%   -49.33%     
===========================================
  Files         166      166               
  Lines       52358    52358               
===========================================
- Hits        48374    22551    -25823     
- Misses       3984    29807    +25823

Flag	Coverage Δ
#multiple	?
#single	43.07% <ø> (-0.03%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/multi.py	34.88% <ø> (-60.71%)	⬇️
pandas/core/strings.py	33% <ø> (-65.59%)	⬇️
pandas/core/indexes/base.py	56.54% <ø> (-39.77%)	⬇️
pandas/io/formats/latex.py	0% <0%> (-100%)	⬇️
pandas/core/categorical.py	0% <0%> (-100%)	⬇️
pandas/io/sas/sas_constants.py	0% <0%> (-100%)	⬇️
pandas/tseries/plotting.py	0% <0%> (-100%)	⬇️
pandas/tseries/converter.py	0% <0%> (-100%)	⬇️
pandas/io/formats/html.py	0% <0%> (-99.35%)	⬇️
pandas/core/groupby/categorical.py	0% <0%> (-95.46%)	⬇️
... and 126 more

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[codecov]

Codecov Report

Merging #24731 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #24731   +/-   ##
=======================================
  Coverage   92.39%   92.39%           
=======================================
  Files         166      166           
  Lines       52358    52358           
=======================================
  Hits        48374    48374           
  Misses       3984     3984

Flag	Coverage Δ
#multiple	90.81% <ø> (ø)	⬆️
#single	43.07% <ø> (-0.03%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/multi.py	95.58% <ø> (ø)	⬆️
pandas/core/strings.py	98.58% <ø> (ø)	⬆️
pandas/core/indexes/base.py	96.3% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fdc4db2...7a8d8b2. Read the comment docs.

[jreback]

thanks @topper-123