Add support for c++17 nested namespaces

For example:
  namespace A::B { ... }
which is the equivalent to C++98 namespaces:
  namespace A { namespace B { ... } }

Examples/test-suite/cpp17_nested_namespaces.i

@@ -0,0 +1,199 @@
+%module cpp17_nested_namespaces
+// Tests c++17 style nested namespaces
+// Tests are designed so that code compiles with C++98 compilers
+
+#define CPP17 1
+%{
+#if __cplusplus >= 201703L
+#define CPP17 1
+#endif
+%}
+
+%inline %{
+// Tests with namespaces already defined using C++98 style (non-nested) namespaces
+namespace A1 {
+  struct A1Struct {
+    void A1Method() {}
+  };
+  namespace B1 {
+    struct B1Struct {
+      void B1Method() {}
+    };
+  }
+}
+#if defined(CPP17)
+namespace A1::B1 {
+#else
+namespace A1 {
+ namespace B1 {
+#endif
+  A1Struct createA1Struct() { return ::A1::A1Struct(); }
+  B1Struct createB1Struct() { return ::A1::B1::B1Struct(); }
+#if !defined(CPP17)
+ }
+}
+#else
+}
+#endif
+
+namespace A1 {
+  namespace B1 {
+    namespace C1 {
+      struct C1Struct {
+        void C1Method() {}
+      };
+    }
+  }
+}
+
+#if defined(CPP17)
+namespace A1::B1::C1 {
+#else
+namespace A1 {
+ namespace B1 {
+  namespace C1 {
+#endif
+  C1Struct createC1Struct() { return ::A1::B1::C1::C1Struct(); }
+#if !defined(CPP17)
+  }
+ }
+}
+#else
+}
+#endif
+%}
+
+%inline %{
+// Tests with namespaces already defined using C++17 style (nested) namespaces
+#if defined(CPP17)
+namespace A2::B2 {
+#else
+namespace A2 {
+ namespace B2 {
+#endif
+  struct B2Struct {
+    void B2Method() {}
+  };
+#if !defined(CPP17)
+ }
+}
+#else
+}
+#endif
+
+#if defined(CPP17)
+namespace A2::B2 {
+#else
+namespace A2 {
+ namespace B2 {
+#endif
+  B2Struct createB2Struct() { return ::A2::B2::B2Struct(); }
+#if !defined(CPP17)
+ }
+}
+#else
+}
+#endif
+
+#if defined(CPP17)
+namespace A2::B2::C2 {
+#else
+namespace A2 {
+ namespace B2 {
+  namespace C2 {
+#endif
+    struct C2Struct {
+      void C2Method() {}
+    };
+#if !defined(CPP17)
+  }
+ }
+}
+#else
+}
+#endif
+
+#if defined(CPP17)
+namespace A2::B2::C2 {
+#else
+namespace A2 {
+ namespace B2 {
+  namespace C2 {
+#endif
+  C2Struct createC2Struct() { return ::A2::B2::C2::C2Struct(); }
+#if !defined(CPP17)
+  }
+ }
+}
+#else
+}
+#endif
+%}
+
+
+%inline %{
+// Tests with namespaces already defined using C++17 style (nested) namespaces to 3 levels
+#if defined(CPP17)
+namespace A3::B3::C3 {
+#else
+namespace A3 {
+ namespace B3 {
+  namespace C3 {
+#endif
+    struct C3Struct {
+      void C3Method() {}
+    };
+#if !defined(CPP17)
+  }
+ }
+}
+#else
+}
+#endif
+
+#if defined(CPP17)
+namespace A3::B3::C3 {
+#else
+namespace A3 {
+ namespace B3 {
+  namespace C3 {
+#endif
+  C3Struct createC3Struct() { return ::A3::B3::C3::C3Struct(); }
+#if !defined(CPP17)
+  }
+ }
+}
+#else
+}
+#endif
+
+#if defined(CPP17)
+namespace A3::B3 {
+#else
+namespace A3 {
+ namespace B3 {
+#endif
+  struct B3Struct {
+    void B3Method() {}
+  };
+#if !defined(CPP17)
+ }
+}
+#else
+}
+#endif
+
+#if defined(CPP17)
+namespace A3::B3 {
+#else
+namespace A3 {
+ namespace B3 {
+#endif
+  B3Struct createB3Struct() { return ::A3::B3::B3Struct(); }
+#if !defined(CPP17)
+ }
+}
+#else
+}
+#endif
+%}