remove some ostringstreams

[neheb]

Looks like they're slow under Cygwin.

Maybe fixes #3153

[codecov]

Codecov Report

Attention: Patch coverage is 58.98876% with 73 lines in your changes missing coverage. Please review.

Project coverage is 64.01%. Comparing base (ef6aa48) to head (d8bc93d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nikonmn_int.cpp	43.75%	18 Missing and 9 partials ⚠️
src/properties.cpp	0.00%	10 Missing ⚠️
src/convert.cpp	70.83%	7 Missing ⚠️
src/pngchunk_int.cpp	0.00%	6 Missing ⚠️
src/samsungmn_int.cpp	0.00%	4 Missing ⚠️
src/casiomn_int.cpp	0.00%	3 Missing ⚠️
src/datasets.cpp	76.92%	2 Missing and 1 partial ⚠️
src/panasonicmn_int.cpp	40.00%	3 Missing ⚠️
include/exiv2/value.hpp	50.00%	0 Missing and 2 partials ⚠️
src/canonmn_int.cpp	89.47%	2 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3157      +/-   ##
==========================================
- Coverage   64.18%   64.01%   -0.18%     
==========================================
  Files         104      104              
  Lines       22126    21838     -288     
  Branches    10832    10694     -138     
==========================================
- Hits        14202    13980     -222     
+ Misses       5708     5644      -64     
+ Partials     2216     2214       -2     

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