rbindlist: pre-convert column names to UTF-8

[aitap]

If the return value is not protected, ENC2UTF8() may end up returning a different CHARSXP pointer after the original one is garbage-collected, causing us to lose the TRUELENGTH mark. Solution: store the names of every list item in a separate list.

Fixes: #7452

#6694 will depend on this because it needs a few more changes to memory management inside rbindlist. Can instead be integrated into #6694.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.04%. Comparing base (2654599) to head (1c025c0).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7461   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files          87       87           
  Lines       16689    16694    +5     
=======================================
+ Hits        16529    16534    +5     
  Misses        160      160           

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[github-actions]

No obvious timing issues in HEAD=fix-7452

Generated via commit c8d051e

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

Task	Duration
R setup and installing dependencies	5 minutes and 13 seconds
Installing different package versions	10 minutes and 31 seconds
Running and plotting the test cases	2 minutes and 39 seconds

[ben-schwen]

LGTM, TY