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/canvasrenderingcontext2d.rs

@@ -51,7 +51,7 @@ use std::sync::Arc;
 use unpremultiplytable::UNPREMULTIPLY_TABLE;
 
 #[must_root]
-#[derive(Clone, HeapSizeOf, JSTraceable)]
+#[derive(Clone, JSTraceable, MallocSizeOf)]
 #[allow(dead_code)]
 enum CanvasFillOrStrokeStyle {
     Color(RGBA),
@@ -63,12 +63,12 @@ enum CanvasFillOrStrokeStyle {
 #[dom_struct]
 pub struct CanvasRenderingContext2D {
     reflector_: Reflector,
-    #[ignore_heap_size_of = "Defined in ipc-channel"]
+    #[ignore_malloc_size_of = "Defined in ipc-channel"]
     ipc_renderer: IpcSender<CanvasMsg>,
     /// For rendering contexts created by an HTML canvas element, this is Some,
     /// for ones created by a paint worklet, this is None.
     canvas: Option<Dom<HTMLCanvasElement>>,
-    #[ignore_heap_size_of = "Arc"]
+    #[ignore_malloc_size_of = "Arc"]
     image_cache: Arc<ImageCache>,
     /// Any missing image URLs.
     missing_image_urls: DomRefCell<Vec<ServoUrl>>,
@@ -81,7 +81,7 @@ pub struct CanvasRenderingContext2D {
 }
 
 #[must_root]
-#[derive(Clone, HeapSizeOf, JSTraceable)]
+#[derive(Clone, JSTraceable, MallocSizeOf)]
 struct CanvasContextState {
     global_alpha: f64,
     global_composition: CompositionOrBlending,