std.string: clean imports

~~Needs https://github.com/D-Programming-Language/dmd/pull/4122~~ merged
[Issue 13253 - use more scoped imports in phobos](https://issues.dlang.org/show_bug.cgi?id=13253)
See also: #2661 
Well if `std.uni` already covers exactly this type of cases I see no problem here - and at quick glance it does that and much more.