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/inline.rs

@@ -17,7 +17,6 @@ use layout_debug;
 use model::IntrinsicISizesContribution;
 use text;
 
-use collections::VecDeque;
 use geom::{Point2D, Rect, Size2D};
 use gfx::display_list::OpaqueNode;
 use gfx::font::FontMetrics;
@@ -25,6 +24,7 @@ use gfx::font_context::FontContext;
 use gfx::text::glyph::CharIndex;
 use gfx::text::text_run::TextRun;
 use std::cmp::max;
+use std::collections::VecDeque;
 use std::fmt;
 use std::mem;
 use std::sync::Arc;

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());

components/layout/lib.rs

@@ -10,7 +10,6 @@
 #![feature(plugin)]
 #![feature(std_misc)]
 #![feature(str_char)]
-#![feature(thread_local)]
 #![feature(unsafe_no_drop_flag)]
 
 #![deny(unsafe_code)]
@@ -40,7 +39,6 @@ extern crate alloc;
 extern crate azure;
 extern crate canvas_traits;
 extern crate clock_ticks;
-extern crate collections;
 extern crate cssparser;
 extern crate encoding;
 extern crate geom;