std.string: clean imports

[Issue 13253 - use more scoped imports in phobos](https://issues.dlang.org/show_bug.cgi?id=13253)
See also: #2661 
I *think* I understand why you're doing this, but are you sure it actually works? Wouldn't the alias *always* instantiate `icmpImpl!()`? AFAIK, the compiler doesn't delay instantiation of an alias if it references an instantiated symbol, but I could be wrong.