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/computed/mod.rs

@@ -397,9 +397,7 @@ impl From<GreaterThanOrEqualToOneNumber> for CSSFloat {
 }
 
 #[allow(missing_docs)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, ComputeSquaredDistance, Copy, Debug, PartialEq, ToCss)]
+#[derive(Clone, ComputeSquaredDistance, Copy, Debug, MallocSizeOf, PartialEq, ToCss)]
 pub enum NumberOrPercentage {
     Percentage(Percentage),
     Number(Number),
@@ -468,7 +466,7 @@ pub type LengthOrPercentageOrNumber = Either<Number, LengthOrPercentage>;
 pub type NonNegativeLengthOrPercentageOrNumber = Either<NonNegativeNumber, NonNegativeLengthOrPercentage>;
 
 #[allow(missing_docs)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 #[derive(Clone, ComputeSquaredDistance, Copy, Debug, PartialEq)]
 /// A computed cliprect for clip and image-region
 pub struct ClipRect {
@@ -550,10 +548,10 @@ pub type ColorOrAuto = Either<Color, Auto>;
 
 /// The computed value of a CSS `url()`, resolved relative to the stylesheet URL.
 #[cfg(feature = "servo")]
-#[derive(Clone, Debug, Deserialize, HeapSizeOf, PartialEq, Serialize)]
+#[derive(Clone, Debug, Deserialize, MallocSizeOf, PartialEq, Serialize)]
 pub enum ComputedUrl {
     /// The `url()` was invalid or it wasn't specified by the user.
-    Invalid(Arc<String>),
+    Invalid(#[ignore_malloc_size_of = "Arc"] Arc<String>),
     /// The resolved `url()` relative to the stylesheet URL.
     Valid(ServoUrl),
 }