std.numeric: clean import & CustomFloat printing fix

Blocker for #2728 
Sounds like a bug in `std.format` or maybe dmd. According to the language, writing `void toString()(...)` should be identical to writing `template toString() { void toString(...) ... }`. IOW, it's just syntactic sugar for an eponymous template. If this doesn't work, it's either a compiler bug or a Phobos bug.