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 
I see. I'm kinda hesitant about removing unittests, in case there are potential bugs caught here that aren't caught elsewhere. This unittest in particular looks like it's testing quite specifically for cases that are likely to break if `icmp` has a bug, so I'm rather reluctant to just delete them.