usable @nogc Exceptions with Mir Runtime

[Oleg]

Thanks for your work!

> Example
> =======================
> ///
> @safe pure nothrow @nogc
> unittest
> {
>     import mir.exception;
>     import mir.format;
>     try throw new MirException(stringBuf() << "Hi D" << 2 << 
> "!" << getData);
>     catch(Exception e) assert(e.msg == "Hi D2!");
> }
>
> =======================

I don't understand why you choose C++ format style instead of 
D-style format?