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/response.rs

@@ -36,7 +36,7 @@ pub struct Response {
     mime_type: DomRefCell<Vec<u8>>,
     body_used: Cell<bool>,
     /// `None` can be considered a StatusCode of `0`.
-    #[ignore_heap_size_of = "Defined in hyper"]
+    #[ignore_malloc_size_of = "Defined in hyper"]
     status: DomRefCell<Option<StatusCode>>,
     raw_status: DomRefCell<Option<(u16, Vec<u8>)>>,
     response_type: DomRefCell<DOMResponseType>,
@@ -44,7 +44,7 @@ pub struct Response {
     url_list: DomRefCell<Vec<ServoUrl>>,
     // For now use the existing NetTraitsResponseBody enum
     body: DomRefCell<NetTraitsResponseBody>,
-    #[ignore_heap_size_of = "Rc"]
+    #[ignore_malloc_size_of = "Rc"]
     body_promise: DomRefCell<Option<(Rc<Promise>, BodyType)>>,
 }