Replace all uses of the heapsize crate with malloc_size_of.

Servo currently uses `heapsize`, but Stylo/Gecko use `malloc_size_of`.
`malloc_size_of` is better -- it handles various cases that `heapsize` does not
-- so this patch changes Servo to use `malloc_size_of`.

This patch makes the following changes to the `malloc_size_of` crate.

- Adds `MallocSizeOf` trait implementations for numerous types, some built-in
  (e.g. `VecDeque`), some external and Servo-only (e.g. `string_cache`).

- Makes `enclosing_size_of_op` optional, because vanilla jemalloc doesn't
  support that operation.

- For `HashSet`/`HashMap`, falls back to a computed estimate when
  `enclosing_size_of_op` isn't available.

- Adds an extern "C" `malloc_size_of` function that does the actual heap
  measurement; this is based on the same functions from the `heapsize` crate.

This patch makes the following changes elsewhere.

- Converts all the uses of `heapsize` to instead use `malloc_size_of`.

- Disables the "heapsize"/"heap_size" feature for the external crates that
  provide it.

- Removes the `HeapSizeOf` implementation from `hashglobe`.

- Adds `ignore` annotations to a few `Rc`/`Arc`, because `malloc_size_of`
  doesn't derive those types, unlike `heapsize`.

components/script/dom/bindings/num.rs

@@ -4,7 +4,7 @@
 
 //! The `Finite<T>` struct.
 
-use heapsize::HeapSizeOf;
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use num_traits::Float;
 use std::default::Default;
 use std::ops::Deref;
@@ -41,9 +41,9 @@ impl<T: Float> Deref for Finite<T> {
     }
 }
 
-impl<T: Float + HeapSizeOf> HeapSizeOf for Finite<T> {
-    fn heap_size_of_children(&self) -> usize {
-        (**self).heap_size_of_children()
+impl<T: Float + MallocSizeOf> MallocSizeOf for Finite<T> {
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        (**self).size_of(ops)
     }
 }