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/values/mod.rs

@@ -50,7 +50,7 @@ pub fn serialize_dimension<W>(value: CSSFloat, unit: &str, dest: &mut W)
 }
 
 /// Convenience void type to disable some properties and values through types.
-#[cfg_attr(feature = "servo", derive(Deserialize, HeapSizeOf, Serialize))]
+#[cfg_attr(feature = "servo", derive(Deserialize, MallocSizeOf, Serialize))]
 #[derive(Clone, Copy, Debug, PartialEq, ToComputedValue, ToCss)]
 pub enum Impossible {}
 
@@ -64,9 +64,7 @@ impl Parse for Impossible {
 }
 
 /// A struct representing one of two kinds of values.
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Animate, Clone, ComputeSquaredDistance, Copy)]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, MallocSizeOf)]
 #[derive(PartialEq, ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
 pub enum Either<A, B> {
     /// The first value.
@@ -96,9 +94,7 @@ impl<A: Parse, B: Parse> Parse for Either<A, B> {
 }
 
 /// <https://drafts.csswg.org/css-values-4/#custom-idents>
-#[derive(Clone, Debug, Eq, Hash, PartialEq, ToComputedValue)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, Eq, Hash, MallocSizeOf, PartialEq, ToComputedValue)]
 pub struct CustomIdent(pub Atom);
 
 impl CustomIdent {
@@ -127,9 +123,7 @@ impl ToCss for CustomIdent {
 }
 
 /// <https://drafts.csswg.org/css-animations/#typedef-keyframes-name>
-#[derive(Clone, Debug, ToComputedValue)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, MallocSizeOf, ToComputedValue)]
 pub enum KeyframesName {
     /// <custom-ident>
     Ident(CustomIdent),