Audit and reduce unstable usage in layout

Reasons behind existing unstable features:

alloc:

 - `make_unique()`
 - direct calls into `heap::allocate()`, etc
 - `boxed::into_raw()` (naming)

collections:

 - `slice_chars()` (needs to prove its worth)

core:

 - lots and lots of pointer manip
 - `raw` stuff

std_misc:

 - Handle stuff
 - hasher stuff

str_char:

 - CharRange

components/layout/layout_task.rs

@@ -861,7 +861,7 @@ impl LayoutTask {
 
                 if opts::get().dump_display_list {
                     println!("#### start printing display list.");
-                    stacking_context.print(String::from_str("#"));
+                    stacking_context.print("#".to_owned());
                 }
 
                 rw_data.stacking_context = Some(stacking_context.clone());