std.experimental.logger formal review round 3

[Martin Nowak via Digitalmars-d]

On 10/30/2014 12:11 AM, Robert burner Schadek wrote:
> That can actually be added to the current state of std.logger without
> breaking any api. The string mixin, version string matching isn't really
> pretty, but it gets the job done. Anyway IMO your approach presented
> here and my approach can go hand in hang. Yours should be propagated as
> the idiomatic way and if you really need the crowbar, which you need
> sometimes, use StdLoggerDisableXXXXX.

I just found a very compelling solution to the LogLevel disabling problem.

Basically we can declare a logLevel for each module or package and let 
the logger do a reverse look up.

```d
module mymod;

import logger;

// can be anything that converts to a LogLevel
// calls with lower log level can be optimized away if this is a compile 
time constant
// runtime values also work nicely but incur a small overhead
// if this declaration is not present, logLevel from the package is used 
or a default LogLevel.
enum logLevel = LogLevel.critical;

void foo()
{
     info("information from foo"); // optimized out
     fatal("error"); // works
}
```

https://gist.github.com/MartinNowak/443f11aa017d14007c35

There is a tiny gotcha, this works because the logger module imports the 
callee module. So we'd need to avoid module constructors in the logger 
module or use a workaround.