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/style/stylesheets/stylesheet.rs

@@ -38,7 +38,7 @@ pub struct UserAgentStylesheets {
 /// A set of namespaces applying to a given stylesheet.
 ///
 /// The namespace id is used in gecko
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Debug, Default, MallocSizeOf)]
 #[allow(missing_docs)]
 pub struct Namespaces {
     pub default: Option<(Namespace, NamespaceId)>,
@@ -275,9 +275,9 @@ impl StylesheetInDocument for Stylesheet {
 /// A simple wrapper over an `Arc<Stylesheet>`, with pointer comparison, and
 /// suitable for its use in a `StylesheetSet`.
 #[derive(Clone)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub struct DocumentStyleSheet(
-    #[cfg_attr(feature = "servo", ignore_heap_size_of = "Arc")]
+    #[cfg_attr(feature = "servo", ignore_malloc_size_of = "Arc")]
     pub Arc<Stylesheet>
 );