Declare variables at point of use in assign.c

[badasahog]

Data desegregation + minor formatting improvements + constness improvements

progress towards #6971

As an aside, I've been awaiting review for other PRs, #7248, #7247, #7244, #7238

[github-actions]

• HEAD=dataCodeIntegration3 much faster for as.data.table.array improved in #7019
  

Generated via commit 153815d

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	6 minutes and 26 seconds
Installing different package versions	11 minutes and 55 seconds
Running and plotting the test cases	5 minutes and 22 seconds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.85%. Comparing base (479ff3a) to head (153815d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7303   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files          87       87           
  Lines       17127    17128    +1     
=======================================
+ Hits        16931    16932    +1     
  Misses        196      196           

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[rikivillalba]

Might conflict with 2db2a90 #7343

[ben-schwen]

TY!