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/globalscope.rs

@@ -72,22 +72,22 @@ pub struct GlobalScope {
     console_timers: DomRefCell<HashMap<DOMString, u64>>,
 
     /// For providing instructions to an optional devtools server.
-    #[ignore_heap_size_of = "channels are hard"]
+    #[ignore_malloc_size_of = "channels are hard"]
     devtools_chan: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
 
     /// For sending messages to the memory profiler.
-    #[ignore_heap_size_of = "channels are hard"]
+    #[ignore_malloc_size_of = "channels are hard"]
     mem_profiler_chan: mem::ProfilerChan,
 
     /// For sending messages to the time profiler.
-    #[ignore_heap_size_of = "channels are hard"]
+    #[ignore_malloc_size_of = "channels are hard"]
     time_profiler_chan: time::ProfilerChan,
 
     /// A handle for communicating messages to the constellation thread.
-    #[ignore_heap_size_of = "channels are hard"]
+    #[ignore_malloc_size_of = "channels are hard"]
     script_to_constellation_chan: ScriptToConstellationChan,
 
-    #[ignore_heap_size_of = "channels are hard"]
+    #[ignore_malloc_size_of = "channels are hard"]
     scheduler_chan: IpcSender<TimerSchedulerMsg>,
 
     /// <https://html.spec.whatwg.org/multipage/#in-error-reporting-mode>
@@ -108,7 +108,7 @@ pub struct GlobalScope {
     /// same microtask queue.
     ///
     /// <https://html.spec.whatwg.org/multipage/#microtask-queue>
-    #[ignore_heap_size_of = "Rc<T> is hard"]
+    #[ignore_malloc_size_of = "Rc<T> is hard"]
     microtask_queue: Rc<MicrotaskQueue>,
 }