std.numeric: clean import & CustomFloat printing fix

Blocker for #2728 
Why is this a template nested inside a template? It almost certainly isn't worth complicating the code like this just to maybe avoid one single import from happening that is already inside a template instance. Moreover, both of the imports are for std.format. Thus, you only gain something once the compiler implements lazy semantic analysis. But then, the whole thing will probably superfluous superfluous anyway, with `CustomFloat` being a template. Also note that template instances aren't free either.