implement Clone for Box<str>, closes #27323

This is a minor [breaking-change], as it changes what
`boxed_str.to_owned()` does (previously it would deref to `&str` and
call `to_owned` on that to get a `String`). However `Box<str>` is such an
exceptionally rare type that this is not expected to be a serious
concern. Also a `Box<str>` can be freely converted to a `String` to
obtain the previous behaviour anyway.

src/liballoc/lib.rs

@@ -92,6 +92,7 @@
 #![feature(unsafe_no_drop_flag, filling_drop)]
 #![feature(unsize)]
 #![feature(core_slice_ext)]
+#![feature(core_str_ext)]
 
 #![cfg_attr(test, feature(test, alloc, rustc_private, box_raw))]
 #![cfg_attr(all(not(feature = "external_funcs"), not(feature = "external_crate")),