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 think these functions should be mentioned in the ddoc header in full (i.e., `std.uni.icmp` instead of just `icmp`), with hyperlinks to the "correct" module where they are. The reason is that many users are going to browse the docs for `std.string`, and then wonder why many fundamental string functions are "missing". Listing external string-related functions in the header will point them in the right direction.