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/stylesheets.rs

@@ -24,7 +24,7 @@ use stylesheets::{CssRule, StylesheetInDocument};
 /// element is determined by whether the invalidation is stored in the
 /// StylesheetInvalidationSet's invalid_scopes or invalid_elements table.
 #[derive(Debug, Eq, Hash, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 enum Invalidation {
     /// An element with a given id.
     ID(Atom),
@@ -75,7 +75,7 @@ impl Invalidation {
 ///
 /// TODO(emilio): We might be able to do the same analysis for media query
 /// changes too (or even selector changes?).
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub struct StylesheetInvalidationSet {
     /// The subtrees we know we have to restyle so far.
     invalid_scopes: FnvHashSet<Invalidation>,