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/mutationobserver.rs

@@ -24,7 +24,7 @@ use std::rc::Rc;
 #[dom_struct]
 pub struct MutationObserver {
     reflector_: Reflector,
-    #[ignore_heap_size_of = "can't measure Rc values"]
+    #[ignore_malloc_size_of = "can't measure Rc values"]
     callback: Rc<MutationCallback>,
     record_queue: DomRefCell<Vec<DomRoot<MutationRecord>>>,
 }
@@ -35,13 +35,13 @@ pub enum Mutation<'a> {
                 prev: Option<&'a Node>, next: Option<&'a Node> },
 }
 
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 pub struct RegisteredObserver {
     observer: DomRoot<MutationObserver>,
     options: ObserverOptions,
 }
 
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 pub struct ObserverOptions {
     attribute_old_value: bool,
     attributes: bool,