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/servo_arc/lib.rs

@@ -27,8 +27,6 @@ extern crate nodrop;
 #[cfg(feature = "servo")] extern crate serde;
 extern crate stable_deref_trait;
 
-#[cfg(feature = "servo")]
-use heapsize::HeapSizeOf;
 use nodrop::NoDrop;
 #[cfg(feature = "servo")]
 use serde::{Deserialize, Serialize};
@@ -476,15 +474,6 @@ impl<T: ?Sized> AsRef<T> for Arc<T> {
 unsafe impl<T: ?Sized> StableDeref for Arc<T> {}
 unsafe impl<T: ?Sized> CloneStableDeref for Arc<T> {}
 
-// This is what the HeapSize crate does for regular arc, but is questionably
-// sound. See https://github.com/servo/heapsize/issues/37
-#[cfg(feature = "servo")]
-impl<T: HeapSizeOf> HeapSizeOf for Arc<T> {
-    fn heap_size_of_children(&self) -> usize {
-        (**self).heap_size_of_children()
-    }
-}
-
 #[cfg(feature = "servo")]
 impl<'de, T: Deserialize<'de>> Deserialize<'de> for Arc<T>
 {