core/string: fix some forgotten `self`
core/string: fix some forgotten `self`

jgaskins/opal,opal/opal,Ajedi32/opal,Flikofbluelight747/opal,opal/opal,iliabylich/opal,merongivian/opal,charliesome/opal,Ajedi32/opal,suyesh/opal,wied03/opal,c42engineering/opal,gausie/opal,merongivian/opal,Mogztter/opal,wied03/opal,kachick/opal,kachick/opal,catprintlabs/opal,Ajedi32/opal,Flikofbluelight747/opal,suyesh/opal,Mogztter/opal,catprintlabs/opal,opal/opal,jannishuebl/opal,c42engineering/opal,Mogztter/opal,castwide/opal,merongivian/opal,gabrielrios/opal,iliabylich/opal,wied03/opal,gausie/opal,Flikofbluelight747/opal,catprintlabs/opal,jgaskins/opal,bbatsov/opal,jannishuebl/opal,kachick/opal,bbatsov/opal,gabrielrios/opal,castwide/opal,suyesh/opal,gabrielrios/opal,castwide/opal,fazibear/opal,fazibear/opal,jannishuebl/opal,Mogztter/opal,fazibear/opal,domgetter/opal,opal/opal,domgetter/opal,iliabylich/opal,bbatsov/opal,jgaskins/opal,gausie/opal,charliesome/opal,c42engineering/opal,kachick/opal