Add KeyValues.Export() method

[Wend4r]

I need this method to work safely among methodmaps that legacy KeyValues without high positions access.

For example,

methodmap X < KeyValues
{
	// ...

	public SubX GetSubX()
	{
		this.JumpToKey("sub");

		KeyValues hSubValues = this.Export();

		this.GoBack();

		return view_as<SubX>(hSubValues);
	}
};

methodmap SubX < KeyValues
{
	// ...
};

Would be correct for SubX not to be able jump to X with this.GoBack() and changed it. KeyValues.Export() solves this problem.