wstr: Implement `ToOwned` for `WStr`

Similarly to how `str` implements `ToOwned`, and `From<&str> for String`
forwards to `to_owned()`.

This will allow defining `Cow<WStr>`.

wstr/src/buf.rs

@@ -361,9 +361,7 @@ impl DerefMut for WString {
 impl<'a> From<&'a WStr> for WString {
     #[inline]
     fn from(s: &'a WStr) -> Self {
-        let mut buf = Self::new();
-        buf.push_str(s);
-        buf
+        s.to_owned()
     }
 }
 

wstr/src/ptr.rs

@@ -1,7 +1,8 @@
+use alloc::borrow::ToOwned;
 use core::ops::Range;
 use core::ptr::{slice_from_raw_parts, slice_from_raw_parts_mut};
 
-use super::Units;
+use super::{Units, WString};
 
 #[cfg(not(any(target_pointer_width = "32", target_pointer_width = "64")))]
 compile_error!("WStr only supports 32-bits and 64-bits targets");
@@ -46,6 +47,16 @@ pub struct WStr {
     _repr: [()],
 }
 
+impl ToOwned for WStr {
+    type Owned = WString;
+
+    fn to_owned(&self) -> Self::Owned {
+        let mut buf = WString::new();
+        buf.push_str(self);
+        buf
+    }
+}
+
 /// Convenience method to turn a `&T` into a `*mut T`.
 #[inline]
 pub fn ptr_mut<T: ?Sized>(t: &T) -> *mut T {