Replace StrAllocating.into_owned() with .into_string()

We already have into_string(), but it was implemented in terms of
into_owned(). Flip it around and deprecate into_owned().

Remove a few spurious calls to .into_owned() that existed in libregex
and librustdoc.

src/librustdoc/html/format.rs

@@ -356,7 +356,7 @@ impl fmt::Show for clean::Type {
                                     }
                                 }
                            }
-                           ret.into_owned()
+                           ret
                        })
             }
             clean::Proc(ref decl) => {