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/selector_map.rs

@@ -94,9 +94,7 @@ pub trait SelectorMapEntry : Sized + Clone {
 /// * https://bugzilla.mozilla.org/show_bug.cgi?id=681755
 ///
 /// TODO: Tune the initial capacity of the HashMap
-#[derive(Debug)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Debug, MallocSizeOf)]
 pub struct SelectorMap<T: 'static> {
     /// A hash from an ID to rules which contain that ID selector.
     pub id_hash: MaybeCaseInsensitiveHashMap<Atom, SmallVec<[T; 1]>>,
@@ -465,9 +463,7 @@ fn find_bucket<'a>(mut iter: SelectorIter<'a, SelectorImpl>) -> Bucket<'a> {
 }
 
 /// Wrapper for PrecomputedHashMap that does ASCII-case-insensitive lookup in quirks mode.
-#[derive(Debug)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Debug, MallocSizeOf)]
 pub struct MaybeCaseInsensitiveHashMap<K: PrecomputedHash + Hash + Eq, V: 'static>(PrecomputedDiagnosticHashMap<K, V>);
 
 // FIXME(Manishearth) the 'static bound can be removed when