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_traits/values.rs

@@ -415,7 +415,7 @@ macro_rules! __define_css_keyword_enum__add_optional_traits {
     ($name: ident [ $( $css: expr => $variant: ident ),+ ]
                   [ $( $alias: expr => $alias_variant: ident),* ]) => {
         __define_css_keyword_enum__actual! {
-            $name [ Deserialize, Serialize, HeapSizeOf ]
+            $name [ Deserialize, Serialize, MallocSizeOf ]
                   [ $( $css => $variant ),+ ]
                   [ $( $alias => $alias_variant ),* ]
         }
@@ -490,9 +490,7 @@ pub mod specified {
 
     /// Whether to allow negative lengths or not.
     #[repr(u8)]
-    #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-    #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-    #[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd)]
+    #[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq, PartialOrd)]
     pub enum AllowedNumericType {
         /// Allow all kind of numeric values.
         All,