importing of C++ nested generics like std::vector<T>::iterator, using the apostrophe operator

tests/cpp/vector_iterator.nim

@@ -0,0 +1,21 @@
+discard """
+  cmd: "nim cpp $file"
+"""
+
+{.emit: """
+
+template <class T>
+struct Vector {
+  struct Iterator {
+
+  };
+};
+
+""".}
+
+type
+  Vector {.importcpp: "Vector".} [T] = object
+  VectorIterator {.importcpp: "Vector<'2>::Iterator".} [T] = object
+
+var x: VectorIterator[void]
+