std.experimental.logger formal review round 3

[Marco Leise via Digitalmars-d]

This module is going to be the go-to logging in D.
Every other D logging library will extend on it, and libraries
will pass their messages through std.logger's `stdlog`.

It is very important that all of you who plan on using logging
in D or know about specific logger implementations take their
time and see if what they have in mind can be implemented on
the API foundations that std.logger provides. Our community
lacks a real logging guru and needs the collective experience.

If you don't have time to experiment, but believe that some
use case must be covered just describe it in a few words.
(Especially effects on API design, performance or threading.)