Modernise code around iterators & smart pointers

[piponazo]

In this PR my main objective was to cleanup some code around the iterators & smart pointers:

• Try to use auto when we are dealing with iterators since it makes the code much less verbose and without lossing clarity.
• Use std::make_unique whenever possible when dealing with smart pointers. It reduces the amount of code and specially it avoids the usage of new and delete. This follows one of the rules in the C++ Core guidelines.
• I also took the oportunity to simplify other parts of the code as I saw improvements opportunities. I tried to scope each change in a independent & atomic commit.

[codecov]

Codecov Report

Merging #2063 (93fe208) into main (fde0f9e) will decrease coverage by 0.16%.
The diff coverage is 51.29%.

@@            Coverage Diff             @@
##             main    #2063      +/-   ##
==========================================
- Coverage   62.04%   61.88%   -0.17%     
==========================================
  Files          96       96              
  Lines       19194    19054     -140     
  Branches     9833     9780      -53     
==========================================
- Hits        11909    11791     -118     
- Misses       4978     4980       +2     
+ Partials     2307     2283      -24     

Impacted Files	Coverage Δ
include/exiv2/types.hpp	70.58% <ø> (ø)
src/convert.cpp	53.84% <ø> (ø)
src/cr2image.cpp	9.09% <0.00%> (ø)
src/epsimage.cpp	1.83% <0.00%> (+<0.01%)	⬆️
src/gifimage.cpp	21.42% <0.00%> (ø)
src/tgaimage.cpp	8.69% <0.00%> (ø)
src/tiffcomposite_int.hpp	89.18% <ø> (ø)
src/pngimage.cpp	64.04% <33.33%> (+0.16%)	⬆️
src/rw2image.cpp	36.36% <33.33%> (ø)
src/jpgimage.cpp	70.43% <37.50%> (-0.05%)	⬇️
... and 39 more

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