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/applicable_declarations.rs

@@ -36,9 +36,7 @@ const SOURCE_ORDER_MASK: u32 = (1 << SOURCE_ORDER_BITS) - 1;
 const SOURCE_ORDER_MAX: u32 = SOURCE_ORDER_MASK;
 
 /// Stores the source order of a block and the cascade level it belongs to.
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Eq, PartialEq)]
+#[derive(Clone, Copy, Eq, MallocSizeOf, PartialEq)]
 struct SourceOrderAndCascadeLevel(u32);
 
 impl SourceOrderAndCascadeLevel {
@@ -76,13 +74,10 @@ impl Debug for SourceOrderAndCascadeLevel {
 ///
 /// This represents the declarations in a given declaration block for a given
 /// importance.
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
 pub struct ApplicableDeclarationBlock {
     /// The style source, either a style rule, or a property declaration block.
-    #[cfg_attr(feature = "gecko", ignore_malloc_size_of = "contains Arcs")]
-    #[cfg_attr(feature = "servo", ignore_heap_size_of = "Arc")]
+    #[ignore_malloc_size_of = "Arc"]
     pub source: StyleSource,
     /// The source order of the block, and the cascade level it belongs to.
     order_and_level: SourceOrderAndCascadeLevel,