std.array: clean imports

[Issue 13253 - use more scoped imports in phobos](https://issues.dlang.org/show_bug.cgi?id=13253)
See also: #2661 
What about template with internal assert?

```D
template assocArray(Range) if(isInputRange!Range &&
         ElementType!Range.length == 2)
{
    static assert(isTuple!(ElementType!Range), "assocArray: argument must be a range of tuples");
    aut assocArray(Range r)
    {
       ...
    }
}
```
Phobos has a lot of stuff like this.