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/htmlimageelement.rs

@@ -85,7 +85,7 @@ pub struct Descriptor {
     pub den: Option<f64>,
 }
 
-#[derive(Clone, Copy, HeapSizeOf, JSTraceable)]
+#[derive(Clone, Copy, JSTraceable, MallocSizeOf)]
 #[allow(dead_code)]
 enum State {
     Unavailable,
@@ -100,19 +100,19 @@ pub struct Size {
     pub length: Length,
 }
 
-#[derive(Clone, Copy, HeapSizeOf, JSTraceable)]
+#[derive(Clone, Copy, JSTraceable, MallocSizeOf)]
 enum ImageRequestPhase {
     Pending,
     Current
 }
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 #[must_root]
 struct ImageRequest {
     state: State,
     parsed_url: Option<ServoUrl>,
     source_url: Option<DOMString>,
     blocker: Option<LoadBlocker>,
-    #[ignore_heap_size_of = "Arc"]
+    #[ignore_malloc_size_of = "Arc"]
     image: Option<Arc<Image>>,
     metadata: Option<ImageMetadata>,
     final_url: Option<ServoUrl>,
@@ -695,7 +695,7 @@ impl HTMLImageElement {
 
 }
 
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 pub enum ImageElementMicrotask {
     StableStateUpdateImageDataTask {
         elem: DomRoot<HTMLImageElement>,