Add display method to OsStr

Add `display` method to `OsStr` for lossy display of an `OsStr` which may contain invalid unicode.

Invalid Unicode sequences are replaced with `U+FFFD REPLACEMENT CHARACTER`.

This change also makes the `std::ffi::os_str` module public.

tests/rustdoc-js-std/osstring-to-string.js

@@ -4,6 +4,6 @@
 const EXPECTED = {
     'query': 'OsString -> String',
     'others': [
-        { 'path': 'std::ffi::OsString', 'name': 'into_string' },
+        { 'path': 'std::ffi::os_str::OsString', 'name': 'into_string' },
     ]
 };