std.array: clean imports

[Issue 13253 - use more scoped imports in phobos](https://issues.dlang.org/show_bug.cgi?id=13253)
See also: #2661 
I thought about that. But currently, ddoc doesn't really handle eponymous templates very well (it doesn't know to merge the template and function declarations). So you'd still have to document it in the ddoc header anyway.