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 
This stuff has an aliases in this module:
```D
alias icmp = std.uni.icmp;
```
With this aliases selective imports does not compile.