std.experimental.logger formal review round 3

[Dicebot via Digitalmars-d]

> I don't recall off the top of my head some non-template innards 
> actually might require @safe, but apart from that, why not just 
> leave the job to template attribute inference entirely? If 
> somebody wants to log a type with a @system toString in 
> non- at safe code, why not just let them?
>
> David

In my opinion inference is better choice for small building 
blocks (like algorithms). For complete system like logging API 
forcing @safe makes more sense as whatever its internals are, 
exposed API should never be @system