std.string: clean imports

[Issue 13253 - use more scoped imports in phobos](https://issues.dlang.org/show_bug.cgi?id=13253)
See also: #2661 
Ohhh I see what you're doing now. Looks OK then. Though I would rename it `trustedPrintf` so that we know what's going on here. And I'd be careful about who calls it, since `@trusted` can lead to safety issues if misused.