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 
On second thoughts, that might actually break code, because before, user code could just import `std.string` and get `icmp` defined, but without the alias, it doesn't work anymore. :-(