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 
Hmm. Perhaps the solution is to add `icmp` to the ddoc header of this module (properly hyperlinked to `std.uni.icmp`) and delete the alias?