Improve speed of logging methods with an additional "f" suffix

[XSven]

I have removed one level of subroutine calls. Maybe related to #69

[preaction]

Thanks! This looks good as-is, though I think the conditional inside the sub that checks if it's a formatter could be skipped if instead the sub was built via string eval "sub { ... }". Then the conditional would be at "compile" time and not runtime.

If you'd like I can merge this as-is and we can leave micro-optimization as a todo. I'll do that Sunday, April 26 unless you tell me otherwise :)