Optional: Move optional-lite to the include/third-party directory

This fixes the use case of adding the include folder
manually to an outside project.

include/icsneo/third-party/optional-lite/example/04-any-optional-variant.cpp

@@ -0,0 +1,43 @@
+#include "nonstd/any.hpp"
+#include "nonstd/optional.hpp"
+#include "nonstd/variant.hpp"
+
+#include <cassert>
+#include <string>
+
+using namespace nonstd;
+
+int main()
+{
+    std::string hello = "hello, world";
+    {
+        optional< int > var;
+
+        assert( ! var );
+
+        var =   7  ; assert( *var         ==  7  );
+        var =   7  ; assert(  var.value() ==  7  );
+    }{
+        any var;
+
+        assert( ! var.has_value() );
+
+        var =  'v' ; assert( any_cast<char>( var ) == 'v' );
+        var =   7  ; assert( any_cast<int >( var ) ==  7  );
+        var =  42L ; assert( any_cast<long>( var ) == 42L );
+        var = hello; assert( any_cast<std::string>( var ) == hello );
+    }{
+        variant< char, int, long, std::string > var;
+
+        assert( ! var.valueless_by_exception() );
+
+                     assert( get<  0 >( var ) == char() );
+        var =  'v' ; assert( get<char>( var ) == 'v'    );
+        var =   7  ; assert( get<int >( var ) ==  7     );
+        var =  42L ; assert( get<long>( var ) == 42L    );
+        var = hello; assert( get<std::string>( var ) == hello );
+    }
+}
+
+// cl -nologo -EHsc -I../../any-lite/include -I../../optional-lite/include -I../../variant-lite/include 04-any-optional-variant.cpp && 04-any-optional-variant
+// g++ -Wall -I../../any-lite/include -I../../optional-lite/include -I../../variant-lite/include -o 04-any-optional-variant 04-any-optional-variant.cpp && 04-any-optional-variant