Merge pull request #475 from ModProg/const-from_slice_unchecked

Make from_slice_unchecked const

dbus/src/strings.rs

@@ -55,7 +55,7 @@ impl<'m> $t<'m> {
 
     /// This function creates a new instance of this struct, without checking.
     /// It's up to you to guarantee that s ends with a \0 and is valid.
-    pub unsafe fn from_slice_unchecked(s: &'m str) -> $t<'m> {
+    pub const unsafe fn from_slice_unchecked(s: &'m str) -> $t<'m> {
         let ss = s.as_bytes();
         debug_assert!(ss[ss.len()-1] == 0);
         $t(Cow::Borrowed(s))