Auto merge of #39594 - clarcharr:cstr_box, r=aturon

Conversions between CStr, OsStr, Path and boxes

This closes a bit of the inconsistencies between `CStr`, `OsStr`, `Path`, and `str`, allowing people to create boxed versions of DSTs other than `str` and `[T]`.

Full list of additions:
* `Default` for `Box<str>`, `Box<CStr>`, `Box<OsStr>`, and `Box<Path>` (note: `Default` for `PathBuf` is already implemented)
* `CString::into_boxed_c_str` (feature gated)
* `OsString::into_boxed_os_str` (feature gated)
* `Path::into_boxed_path` (feature gated)
* `From<&CStr> for Box<CStr>`
* `From<&OsStr> for Box<OsStr>`
* `From<&Path> for Box<Path>`

This also includes adding the internal methods:
* `sys::*::os_str::Buf::into_box`
* `sys::*::os_str::Slice::{into_box, empty_box}`
* `sys_common::wtf8::Wtf8Buf::into_box`
* `sys_common::wtf8::Wtf8::{into_box, empty_box}`

src/liballoc/boxed.rs

@@ -315,6 +315,14 @@ impl<T> Default for Box<[T]> {
     }
 }
 
+#[stable(feature = "default_box_extra", since = "1.17.0")]
+impl Default for Box<str> {
+    fn default() -> Box<str> {
+        let default: Box<[u8]> = Default::default();
+        unsafe { mem::transmute(default) }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone> Clone for Box<T> {
     /// Returns a new box with a `clone()` of this box's contents.