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/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;