Audit and reduce unstable usage in gfx

Reasons behind existing unstable features:

alloc:

 - weak pointers (may not belong)
 - boxed::into_raw and from_raw (naming)

collections:

 - `Vec.push_all()` (May be replaced)
   - Replace with loop

core:

 - `min_by` (#15311)
 - `PI_2` (naming)

rustc_private:

 - FnVHasher
   - Can be replaced by rust-fnv

std_misc:

 - Hash state stuff (no option here unless we want to stop using it)

str_char:

 - CharRange usage
   - Can be replaced by regular iterators (char_indices)

components/gfx/display_list/mod.rs

@@ -26,7 +26,7 @@ use text::TextRun;
 use azure::azure::AzFloat;
 use azure::azure_hl::Color;
 
-use collections::linked_list::{self, LinkedList};
+use std::collections::linked_list::{self, LinkedList};
 use geom::{Point2D, Rect, SideOffsets2D, Size2D, Matrix2D, Matrix4};
 use geom::approxeq::ApproxEq;
 use geom::num::Zero;
@@ -297,7 +297,7 @@ impl StackingContext {
 
             if opts::get().dump_display_list_optimized {
                 println!("**** optimized display list. Tile bounds: {:?}", tile_bounds);
-                display_list.print_items(String::from_str("*"));
+                display_list.print_items("*".to_owned());
             }
 
             // Sort positioned children according to z-index.
@@ -560,7 +560,7 @@ impl StackingContext {
     pub fn print(&self, mut indentation: String) {
         // We cover the case of an empty string.
         if indentation.len() == 0 {
-            indentation = String::from_str("####");
+            indentation = "####".to_owned();
         }
 
         // We grow the indentation by 4 characters if needed.

components/gfx/display_list/optimizer.rs

@@ -6,7 +6,7 @@
 
 use display_list::{DisplayItem, DisplayList, StackingContext};
 
-use collections::linked_list::LinkedList;
+use std::collections::linked_list::LinkedList;
 use geom::rect::Rect;
 use util::geometry::{self, Au};
 use std::sync::Arc;

components/gfx/lib.rs

@@ -18,7 +18,6 @@ extern crate log;
 
 extern crate azure;
 #[macro_use] extern crate bitflags;
-extern crate collections;
 extern crate geom;
 extern crate layers;
 extern crate libc;