Revert "Delete dead code"

[piponazo]

This reverts commit 1d6bac6.

As discussed in the commit, @mallman is using this functionality in his library.

[codecov]

Codecov Report

Merging #2078 (1f07715) into main (275bcb9) will decrease coverage by 0.22%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2078      +/-   ##
==========================================
- Coverage   62.09%   61.87%   -0.23%     
==========================================
  Files          96       96              
  Lines       19149    19059      -90     
  Branches     9798     9782      -16     
==========================================
- Hits        11891    11792      -99     
- Misses       4963     4984      +21     
+ Partials     2295     2283      -12     

Impacted Files	Coverage Δ
include/exiv2/datasets.hpp	100.00% <ø> (ø)
src/datasets.cpp	93.03% <0.00%> (-2.42%)	⬇️
src/safe_op.hpp	69.23% <0.00%> (-27.65%)	⬇️
include/exiv2/slice.hpp	69.64% <0.00%> (-21.89%)	⬇️
include/exiv2/error.hpp	60.71% <0.00%> (-4.81%)	⬇️
src/utils.cpp	38.46% <0.00%> (-1.93%)	⬇️
include/exiv2/value.hpp	82.68% <0.00%> (-0.56%)	⬇️
src/enforce.hpp	75.00% <0.00%> (+2.77%)	⬆️
src/getopt.cpp	67.30% <0.00%> (+5.76%)	⬆️
include/exiv2/metadatum.hpp	88.88% <0.00%> (+16.16%)	⬆️

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[piponazo]

Thanks @jim-easterbrook for pointing out. As you can see, I have opened a PR for recovering that function.