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/microtask.rs

@@ -21,7 +21,7 @@ use std::mem;
 use std::rc::Rc;
 
 /// A collection of microtasks in FIFO order.
-#[derive(Default, HeapSizeOf, JSTraceable)]
+#[derive(Default, JSTraceable, MallocSizeOf)]
 pub struct MicrotaskQueue {
     /// The list of enqueued microtasks that will be invoked at the next microtask checkpoint.
     microtask_queue: DomRefCell<Vec<Microtask>>,
@@ -29,7 +29,7 @@ pub struct MicrotaskQueue {
     performing_a_microtask_checkpoint: Cell<bool>,
 }
 
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 pub enum Microtask {
     Promise(EnqueuedPromiseCallback),
     MediaElement(MediaElementMicrotask),
@@ -43,9 +43,9 @@ pub trait MicrotaskRunnable {
 }
 
 /// A promise callback scheduled to run during the next microtask checkpoint (#4283).
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 pub struct EnqueuedPromiseCallback {
-    #[ignore_heap_size_of = "Rc has unclear ownership"]
+    #[ignore_malloc_size_of = "Rc has unclear ownership"]
     pub callback: Rc<PromiseJobCallback>,
     pub pipeline: PipelineId,
 }