parsec: use StringLiteralRef in implementation of literalString function
parsec: use StringLiteralRef in implementation of literalString function

as suggested in https://github.com/jckarter/clay/pull/326#discussion_r1362567

jckarter/clay,mario-campos/clay,jckarter/clay,iamrekcah/clay,jckarter/clay,jckarter/clay,mario-campos/clay,iamrekcah/clay,iamrekcah/clay,iamrekcah/clay,mario-campos/clay,mario-campos/clay,jckarter/clay,mario-campos/clay,jckarter/clay,iamrekcah/clay