Simplify DataContainer.assign operation

[charlesbluca]

As pointed out by @quasiben and @jdye64, the DataContainer.assign operation could definitely be simplified. This change it to do the same thing, but with only one getitem call and a rename operation instead of assign (not sure if we should change the name to reflect this). For comparison, here are the graphs for a basic SQL operation:

SELECT
     user_id, SUM(b) AS "S"
FROM user_table_1
GROUP BY user_id

Before this PR:

HighLevelGraph with 20 layers.
<dask.highlevelgraph.HighLevelGraph object at 0x7fbc344ae590>
 0. from_pandas-b5125674539d8b5d1f754260e384e1fa
 1. assign-e29cf71863faee69539b6f78b8dbc8f3
 2. getitem-aa5bcd5e86836ebad1df2a90117e16f6
 3. fillna-94de4188d246caf9e1b4bf4f1debce03
 4. isnull-76ff5f3bc798aa3da45b6b16dae758fc
 5. inv-8c38b3dd8368fbb685b718f17180f00b
 6. aggregate-agg-838eda3f7eb4dbc264011a4f8e14a10b
 7. rename-c2e63c54b4d4fff51825029f94563613
 8. reset_index-3a34f3d8a1856ef12b299845da6afa8c
 9. rename-79dfdb294050405020a476146c8b5845
 10. getitem-be78c3b9d2c681e09cccdfdf783dbd1f
 11. eq-77e1f642c7c2a8e643f4a22b61af4e11
 12. inv-07fa50e82ef9e1e810819771a8e1ce2d
 13. getitem-1cc4911cff51b0537d276b6a9de6ebf7
 14. where-7cc4db419a8a2172e17accd935b851f4
 15. assign-8ac7e56123d06d1204acb6609e9541b6
 16. getitem-0ab1e73633488c2092c98ca87c31333c
 17. getitem-2a55368cca98a96e0d82198cd6bd7802
 18. assign-837d2902d726cbdd2f356860ab530f04
 19. getitem-f7c62a09776201bb03dae99feccd8138

And after:

HighLevelGraph with 18 layers.
<dask.highlevelgraph.HighLevelGraph object at 0x7fdf157973a0>
 0. from_pandas-b5125674539d8b5d1f754260e384e1fa
 1. assign-00613e4c7fd862c515ec9466f5853ab4
 2. getitem-fe8d06a9e480fb219e18ea645619fcf4
 3. fillna-cd6269d74d7fcf966419cb47824e2345
 4. isnull-cfb67e5887841dafb6cb0b31d862f9ec
 5. inv-dadddcdaf4eb1433e56291f05e94a3ac
 6. aggregate-agg-a11343737fc93c7c4013b7e3402a0b9f
 7. rename-b85d14b90d651de541c4894a6734961b
 8. reset_index-6d84f857c2528a09277e0c92323afb46
 9. rename-47e52d1413aef810ff415a357c4f6517
 10. getitem-a5a39823bbc0db290fff73a3f3a13e7f
 11. eq-6aef9b02442f4aeff046edb02b7d965c
 12. inv-24a323e2479dbf18adadac7633a7020b
 13. getitem-95c062ea749e3feb0f95196d85463e6e
 14. where-01585d34c6d654f45baf4cca72d713c1
 15. assign-4428e7c82f5ad753650280b081ebb8ff
 16. getitem-6b0569fa23bbbb31e18bd7dac2a7031d
 17. rename-30fe5b74d8f4f3d9e867782189528611

[charlesbluca]

I feel like the manipulation of mapping and use of internal _frontend_backend_mapping here shows that they could be changed to make this look a little simpler. I can dig into where else they are used and think about this some more

[codecov-commenter]

Codecov Report

Merging #271 (b6ac6b7) into main (1bb37c9) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
+ Coverage   95.74%   95.80%   +0.06%     
==========================================
  Files          64       64              
  Lines        2750     2791      +41     
  Branches      412      424      +12     
==========================================
+ Hits         2633     2674      +41     
  Misses         75       75              
  Partials       42       42              

Impacted Files	Coverage Δ
dask_sql/datacontainer.py	94.11% <100.00%> (+0.07%)	⬆️
dask_sql/physical/rex/core/call.py	99.10% <0.00%> (-0.01%)	⬇️
dask_sql/context.py	99.09% <0.00%> (-0.01%)	⬇️
dask_sql/cmd.py	100.00% <0.00%> (ø)
dask_sql/utils.py	100.00% <0.00%> (ø)
dask_sql/server/responses.py	100.00% <0.00%> (ø)

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