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/net_traits/image_cache.rs

@@ -23,9 +23,9 @@ pub enum CanRequestImages {
 }
 
 /// Indicating either entire image or just metadata availability
-#[derive(Clone, Deserialize, HeapSizeOf, Serialize)]
+#[derive(Clone, Deserialize, MallocSizeOf, Serialize)]
 pub enum ImageOrMetadataAvailable {
-    ImageAvailable(Arc<Image>, ServoUrl),
+    ImageAvailable(#[ignore_malloc_size_of = "Arc"] Arc<Image>, ServoUrl),
     MetadataAvailable(ImageMetadata),
 }
 
@@ -60,14 +60,14 @@ impl ImageResponder {
 }
 
 /// The returned image.
-#[derive(Clone, Debug, Deserialize, HeapSizeOf, Serialize)]
+#[derive(Clone, Debug, Deserialize, MallocSizeOf, Serialize)]
 pub enum ImageResponse {
     /// The requested image was loaded.
-    Loaded(Arc<Image>, ServoUrl),
+    Loaded(#[ignore_malloc_size_of = "Arc"] Arc<Image>, ServoUrl),
     /// The request image metadata was loaded.
     MetadataLoaded(ImageMetadata),
     /// The requested image failed to load, so a placeholder was loaded instead.
-    PlaceholderLoaded(Arc<Image>, ServoUrl),
+    PlaceholderLoaded(#[ignore_malloc_size_of = "Arc"] Arc<Image>, ServoUrl),
     /// Neither the requested image nor the placeholder could be loaded.
     None,
 }
@@ -81,7 +81,7 @@ pub enum ImageState {
 }
 
 /// The unique id for an image that has previously been requested.
-#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, HeapSizeOf, PartialEq, Serialize)]
+#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
 pub struct PendingImageId(pub u64);
 
 #[derive(Debug, Deserialize, Serialize)]