Compiler switch to put generated Haxe classes into a child namespace

Currently hxcpp places many classes in the root C++ namespace. Many of these classes have common names like "String", "File", "Object", and "Class". This creates conflicts between hxcpp and other C++ code. The lack of namespace-compliance also makes hxcpp unreliable for production code, because there is no guarantee that its output will work with existing, namespace-compliant code bases.

Ideally in the long run these classes would be put into a child namespace by default, but since so far hxcpp has been using the root namespace, the only viable solution now would be a compiler switch that toggles which namespace (root or child) the generated Haxe classes are placed in.