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/invalidation/media_queries.rs

@@ -23,9 +23,7 @@ use stylesheets::{NestedRuleIterationCondition, Stylesheet, SupportsRule};
 ///
 /// If this changes, though, we may need to remove the item from the cache if
 /// present before it goes away.
-#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, Eq, Hash, MallocSizeOf, PartialEq)]
 pub struct MediaListKey(usize);
 
 impl MediaListKey {
@@ -53,9 +51,7 @@ impl ToMediaListKey for MediaRule {}
 
 /// A struct that holds the result of a media query evaluation pass for the
 /// media queries that evaluated successfully.
-#[derive(Debug, PartialEq)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Debug, MallocSizeOf, PartialEq)]
 pub struct EffectiveMediaQueryResults {
     /// The set of media lists that matched last time.
     set: FnvHashSet<MediaListKey>,