morestl: add example slide for std::optional

Signed-off-by: Abhishek Lekshmanan <abhishek.lekshmanan@cern.ch>

talk/morelanguage/morestl.tex

@@ -25,6 +25,26 @@ \subsection{More STL}
   \end{block}
 \end{frame}
 
+\begin{frame}[fragile]
+  \frametitlecpp[17]{std::optional}
+  \scriptsize
+  \begin{cppcode}
+    std::optional<Phone> parse\_phone(std::string\_view in)
+    {
+      if (is\_valid\_phone(in)) {
+        return std::optional<Phone>(in);
+      }
+      return std::nullopt; // {} also default constructs to nullopt
+    }
+
+    ...
+    if (auto v = parse\_phone(get\_input());
+        v.is\_valid()) {
+        process\_phone(v.value());
+    }
+  \end{cppcode}
+\end{frame}
+
 \begin{frame}[fragile]
   \frametitlecpp[11]{non-member begin and end}
   \begin{alertblock}{The problem in \cpp98}