OsString could implement the Default trait

[`std::ffi::OsString`](http://doc.rust-lang.org/1.7.0/std/ffi/struct.OsString.html) has an associated function `new() -> OsString`, but no Default impl. Probably just an oversight, and so not RFC-worthy IMHO.
