Make rt.util.utf publicly available

[Eduard Staniloiu]

Hello, everyone.

I've been looking over Phobos' `std.utf` and druntime's 
`rt.util.utf` as there is some code duplication and I would like 
to start removing that.

I want to be able to import some of the existing druntime 
functionality in Phobos, so we can remove the duplication: ex. 
isValidDchar, toUTF*, etc.

I'm not able to do so, since, as far as I can tell, the `rt` is 
private: it's not part of the `druntime/import/` folder.

Could you please tell me what's the reasoning behind this 
decision and what would be the course of action going forward?

Thank you,
Edi