SES-1803 : Add Literals.isTypedLiteral and Literals.isLanguageLiteral

These methods are necessary to easily distinguish typed and language
literals

SES-1916 SES-1917 : Add XMLSchema String and RDF LangString to literals
SES-1803 : Clarify RDF_LANGSTRING_TO_LANG_LITERAL semantics

Also change default to true, to ensure that it is not serialised unless
someone actually turns the setting on, as most concrete syntaxes get
confused if it is serialised.
SES-1916 Remove support for RDF LangString removal option

It should always, in current concrete syntaxes, be removed, so only
provide it for future or third-party use in specific scenarios
SES-1917 : Add xsd:string removal support for tuple and rdf formats