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/document_loader.rs

@@ -14,7 +14,7 @@ use net_traits::request::RequestInit;
 use servo_url::ServoUrl;
 use std::thread;
 
-#[derive(Clone, Debug, HeapSizeOf, JSTraceable, PartialEq)]
+#[derive(Clone, Debug, JSTraceable, MallocSizeOf, PartialEq)]
 pub enum LoadType {
     Image(ServoUrl),
     Script(ServoUrl),
@@ -40,7 +40,7 @@ impl LoadType {
 /// Canary value ensuring that manually added blocking loads (ie. ones that weren't
 /// created via DocumentLoader::fetch_async) are always removed by the time
 /// that the owner is destroyed.
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 #[must_root]
 pub struct LoadBlocker {
     /// The document whose load event is blocked by this object existing.
@@ -81,7 +81,7 @@ impl Drop for LoadBlocker {
     }
 }
 
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 pub struct DocumentLoader {
     resource_threads: ResourceThreads,
     blocking_loads: Vec<LoadType>,