Optimize canonical_lexical/1 implementation on RDF.XSD.Datatypes

lib/rdf/literal/datatype.ex

@@ -401,6 +401,7 @@ defmodule RDF.Literal.Datatype do
 
       defoverridable datatype_id: 1,
                      language: 1,
+                     canonical_lexical: 1,
                      cast: 1,
                      do_cast: 1,
                      equal_value?: 2,

lib/rdf/serializations/turtle_encoder.ex

@@ -298,7 +298,7 @@ defmodule RDF.Turtle.Encoder do
   defp term(%Literal{literal: %datatype{}} = literal, state, _, nesting)
         when datatype in @native_supported_datatypes do
     if Literal.valid?(literal) do
-      literal |> Literal.canonical() |> Literal.lexical()
+      Literal.canonical_lexical(literal)
     else
       typed_literal_term(literal, state, nesting)
     end

lib/rdf/xsd/datatype.ex

@@ -225,6 +225,11 @@ defmodule RDF.XSD.Datatype do
 
       def lexical(%__MODULE__{uncanonical_lexical: lexical}), do: lexical
 
+      @impl RDF.Literal.Datatype
+      def canonical_lexical(%RDF.Literal{literal: literal}), do: canonical_lexical(literal)
+      def canonical_lexical(%__MODULE__{value: value}) when not is_nil(value), do: canonical_mapping(value)
+      def canonical_lexical(_), do: nil
+
       @impl RDF.Literal.Datatype
       def canonical(literal)