C: fix #330 with 5.18. set the PL_utf8_tofold ToCf swash statically

dynamically did not work as the existing swash got re-assigned, and the free
of the static keys failed.
Look now for the ToCf key in utf8::SWASHNEW->save and set PL_utf8_tofold to
this static swash.
Fixes issue 330 on 5.18, fixes the d676db56cf8d regressions.