Speed up backport_linter()

[AshesITR]

A slight speedup by using a more efficient data structure for backport checking.

devtools::load_all()
system.time(lint_package(linters = backport_linter("oldrel-4", except = c("R_user_dir", "str2lang", "str2expression", "deparse1", "...names"))))

# here
#    user  system elapsed 
#  5.667   0.011   5.675 

# main
#    user  system elapsed 
#  6.270   0.020   6.286 

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (828bb9c) 99.41% compared to head (a5096a3) 99.41%.

❗ Current head a5096a3 differs from pull request most recent head d731089. Consider uploading reports for the commit d731089 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2348   +/-   ##
=======================================
  Coverage   99.41%   99.41%           
=======================================
  Files         124      124           
  Lines        5654     5655    +1     
=======================================
+ Hits         5621     5622    +1     
  Misses         33       33           

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[AshesITR]

I'll do some more testing to see if I find an even better solution, but avoiding do.call(rbind, ...) saves almost 20%.