Make to! for same-width string nothrow

Works around issue 5700 - Allow dup in nothrow functions
http://d.puremagic.com/issues/show_bug.cgi?id=5700

As trivial as it gets. Also added unittest.

Suggest looking with "no white" `?w=1` diff mode.

EDIT: EG: https://github.com/D-Programming-Language/phobos/pull/1570/files?w=1