Uniting library traits

[Jesse Phillips]

> So, if it turns out that a particular trait is needed by 
> druntime, it does make some sense to move its implementation to 
> druntime, and then have std.traits publicly import it, but from 
> the public API perspective, it should still be in Phobos, and I 
> don't think that we should be trying to move all of the traits 
> into druntime - just those few that we find that are actually 
> needed there.
>
> - Jonathan M Davis

What about the docs? Should doc generation merge in public import 
docs? Seem generally useful.