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/event.rs

@@ -294,7 +294,7 @@ impl EventMethods for Event {
     }
 }
 
-#[derive(Clone, Copy, HeapSizeOf, PartialEq)]
+#[derive(Clone, Copy, MallocSizeOf, PartialEq)]
 pub enum EventBubbles {
     Bubbles,
     DoesNotBubble
@@ -318,7 +318,7 @@ impl From<EventBubbles> for bool {
     }
 }
 
-#[derive(Clone, Copy, HeapSizeOf, PartialEq)]
+#[derive(Clone, Copy, MallocSizeOf, PartialEq)]
 pub enum EventCancelable {
     Cancelable,
     NotCancelable
@@ -344,7 +344,7 @@ impl From<EventCancelable> for bool {
 
 #[derive(Clone, Copy, Debug, Eq, JSTraceable, PartialEq)]
 #[repr(u16)]
-#[derive(HeapSizeOf)]
+#[derive(MallocSizeOf)]
 pub enum EventPhase {
     None      = EventConstants::NONE,
     Capturing = EventConstants::CAPTURING_PHASE,
@@ -363,7 +363,7 @@ pub enum EventPhase {
 ///
 /// [msg]: https://doc.servo.org/script_traits/enum.ConstellationMsg.html#variant.KeyEvent
 ///
-#[derive(Clone, Copy, HeapSizeOf, JSTraceable, PartialEq)]
+#[derive(Clone, Copy, JSTraceable, MallocSizeOf, PartialEq)]
 pub enum EventDefault {
     /// The default action of the event is allowed (constructor's default)
     Allowed,