Update StringfiableExtension.pm

[mohawk2]

Simplify element_stringify

I didn't want to just blindly commit in case there was some reason it was as it was.

[mohawk2]

The build is failing on Windows, saying:

...
  gmake[3]: Leaving directory 'C:/Users/RUNNER~1/.cpanm/work/1706647351.7240/PDL-2.085/Basic/Gen/Inline'
  "C:\hostedtoolcache\windows\strawberry-perl\5.38.2\x64\perl\bin\perl.exe" -MExtUtils::Command -e cp -- pptemplate ..\..\blib\script\pptemplate
  pl2bat.bat ..\..\blib\script\pptemplate
  Can't execute /c/hostedtoolcache/windows/strawberry-perl/5.38.2/x64/perl/bin/pl2bat.bat.
  gmake[2]: *** [Makefile:433: ..\..\blib\script\pptemplate] Error 29
  gmake[2]: Leaving directory 'C:/Users/RUNNER~1/.cpanm/work/1706647351.7240/PDL-2.085/Basic/Gen'
...

[mohawk2]

The Windows thing I fixed on master by version-pinning to an earlier Perl since 5.40 Strawberry is broken in that regard - see StrawberryPerl/Perl-Dist-Strawberry#174. The MacOS CI I've fixed here at no extra charge, it's caused by Sereal/Sereal#300.