std.experimental.logger formal review round 3

[Dicebot via Digitalmars-d]

I had initial go at cleaning safety attributes : 
https://github.com/Dicebot/phobos/tree/logger-fix-trust

It still needs one druntime PR merged 
(https://github.com/D-Programming-Language/druntime/pull/1009) to 
be done properly but should be ready for destruction.

David snipet results in this after all changes:
std/experimental/logger/core.d(1675): Error: safe function 
'std.experimental.logger.core.Logger.logf!(11, "aaa.d", 
"aaa.main", "void aaa.main() @safe", "aaa", Dangerous).logf' 
cannot call system function 'std.format.formattedWrite!(MsgRange, 
char, Dangerous).formattedWrite'