Stop relying on linking details of std’s default allocator

We’ve been bitten before by symbol names changing:
servo/heapsize#46
and upstream is planning to stop using jemalloc by default:
rust-lang/rust#33082 (comment)

So use the (relatively) new `#[global_allocator]` attribute
to explicitly select the system allocator on Windows
and jemalloc (now in an external crate) on other platforms.
This choice matches current defaults.

Cargo.toml

@@ -8,9 +8,6 @@ publish = false
 [lib]
 path = "lib.rs"
 
-[target.'cfg(windows)'.dependencies]
-kernel32-sys = "0.2.1"
-
 [features]
 servo = ["js", "string_cache", "url", "webrender_api", "xml5ever"]
 

lib.rs

@@ -49,8 +49,6 @@ extern crate euclid;
 extern crate hashglobe;
 #[cfg(feature = "servo")]
 extern crate js;
-#[cfg(target_os = "windows")]
-extern crate kernel32;
 extern crate servo_arc;
 extern crate smallbitvec;
 extern crate smallvec;
@@ -63,8 +61,6 @@ extern crate webrender_api;
 #[cfg(feature = "servo")]
 extern crate xml5ever;
 
-#[cfg(target_os = "windows")]
-use kernel32::{GetProcessHeap, HeapSize, HeapValidate};
 use std::hash::{BuildHasher, Hash};
 use std::mem::size_of;
 use std::ops::Range;
@@ -146,33 +142,6 @@ impl MallocSizeOfOps {
     }
 }
 
-/// Get the size of a heap block.
-#[cfg(not(target_os = "windows"))]
-pub unsafe extern "C" fn malloc_size_of(ptr: *const c_void) -> usize {
-    // The C prototype is `je_malloc_usable_size(JEMALLOC_USABLE_SIZE_CONST void *ptr)`. On some
-    // platforms `JEMALLOC_USABLE_SIZE_CONST` is `const` and on some it is empty. But in practice
-    // this function doesn't modify the contents of the block that `ptr` points to, so we use
-    // `*const c_void` here.
-    extern "C" {
-        #[cfg_attr(any(prefixed_jemalloc, target_os = "macos", target_os = "ios", target_os = "android"),
-                   link_name = "je_malloc_usable_size")]
-        fn malloc_usable_size(ptr: *const c_void) -> usize;
-    }
-    malloc_usable_size(ptr)
-}
-
-/// Get the size of a heap block.
-#[cfg(target_os = "windows")]
-pub unsafe extern "C" fn malloc_size_of(mut ptr: *const c_void) -> usize {
-    let heap = GetProcessHeap();
-
-    if HeapValidate(heap, 0, ptr) == 0 {
-        ptr = *(ptr as *const *const c_void).offset(-1);
-    }
-
-    HeapSize(heap, 0, ptr) as usize
-}
-
 /// Trait for measuring the "deep" heap usage of a data structure. This is the
 /// most commonly-used of the traits.
 pub trait MallocSizeOf {