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_parser.rs

@@ -35,7 +35,7 @@ pub use servo::restyle_damage::ServoRestyleDamage as RestyleDamage;
 pub use gecko::restyle_damage::GeckoRestyleDamage as RestyleDamage;
 
 /// Servo's selector parser.
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub struct SelectorParser<'a> {
     /// The origin of the stylesheet we're parsing.
     pub stylesheet_origin: Origin,
@@ -103,8 +103,7 @@ pub enum PseudoElementCascadeType {
 }
 
 /// A per-functional-pseudo map, from a given pseudo to a `T`.
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(MallocSizeOf)]
 pub struct PerPseudoElementMap<T> {
     entries: [Option<T>; SIMPLE_PSEUDO_COUNT],
 }