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'm not sure about moving the import inside the function body. This changes the ddoc and makes it non-obvious that a tuple is expected (until you run into the compile error). I don't have a good solution either -- currently the language is unable to localize an import required by a sig constraint -- but the tuple requirement should at least be documented in the ddoc header.