add resizeVector and export at C level

[jangorecki]

This C function would be useful for benchmarking for checking impact of new R C API for growable vectors. It could be used

x = rnorm(20)
x = .Call(data.table:::CcopyAsGrowable, x)
.Call(data.table:::CresizeVector, x, 5L)
.Call(data.table:::CresizeVector, x, 10L)

AFAIU resizing from R is currently not possible, so adding this function allows us to do so.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.99%. Comparing base (fc60d3c) to head (e2ba7a2).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/frollapply.c	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7810      +/-   ##
==========================================
- Coverage   99.01%   98.99%   -0.03%     
==========================================
  Files          88       88              
  Lines       17219    17223       +4     
==========================================
  Hits        17050    17050              
- Misses        169      173       +4     

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[github-actions]

• HEAD=resizeVector much slower for as.data.table.array improved in #7019
• HEAD=resizeVector much slower for transform improved in #5493
• HEAD=resizeVector slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit e2ba7a2

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

Task	Duration
R setup and installing dependencies	6 minutes and 28 seconds
Installing different package versions	11 minutes and 49 seconds
Running and plotting the test cases	5 minutes and 27 seconds