std.experimental.logger formal review round 3

[Sönke Ludwig via Digitalmars-d]

I still think that there should be the two predefined log levels "debug" 
(for developer related diagnostics) and "diagnostic" (for end user 
related diagnostics) between "trace" and "info". This is important for 
interoperability of different libraries, so that they have predictable 
debug output.

But independent of that, there should really be a function for safely 
generating the user defined intermediate log levels, maybe like this:

     LogLevel raiseLevel(LogLevel base_level, ubyte increment)
     {
         assert(base_level is a valid base level);
         assert(base_level + increment smaller than the next base level);
         return cast(LogLevel)(base_level + increment);
     }

     // ok
     enum notice = raiseLevel(LogLevel.info, 16);

     // error, overlaps with the next base level
     enum suspicious = raiseLevel(LogLevel.info, 32);

Casting to an enum type is a pretty blunt operation, so it should at 
least be made as safe as possible.