mb_substr: respects 8bit encoding
mb_substr: respects 8bit encoding

ref https://github.com/peachpiecompiler/peachpie/issues/943

peachpiecompiler/peachpie,peachpiecompiler/peachpie,iolevel/peachpie-concept,peachpiecompiler/peachpie,iolevel/peachpie-concept,iolevel/peachpie-concept