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/stylesheets/viewport_rule.rs

@@ -78,7 +78,7 @@ macro_rules! declare_viewport_descriptor_inner {
         $number_of_variants: expr
     ) => {
         #[derive(Clone, Debug, PartialEq)]
-        #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[cfg_attr(feature = "servo", derive(MallocSizeOf))]
         #[allow(missing_docs)]
         pub enum ViewportDescriptor {
             $(
@@ -140,7 +140,7 @@ trait FromMeta: Sized {
 /// * http://dev.w3.org/csswg/css-device-adapt/#min-max-width-desc
 /// * http://dev.w3.org/csswg/css-device-adapt/#extend-to-zoom
 #[derive(Clone, Debug, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 #[allow(missing_docs)]
 pub enum ViewportLength {
     Specified(LengthOrPercentageOrAuto),
@@ -231,7 +231,7 @@ struct ViewportRuleParser<'a, 'b: 'a> {
 }
 
 #[derive(Clone, Debug, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 #[allow(missing_docs)]
 pub struct ViewportDescriptorDeclaration {
     pub origin: Origin,
@@ -330,7 +330,7 @@ impl<'a, 'b, 'i> DeclarationParser<'i> for ViewportRuleParser<'a, 'b> {
 
 /// A `@viewport` rule.
 #[derive(Clone, Debug, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub struct ViewportRule {
     /// The declarations contained in this @viewport rule.
     pub declarations: Vec<ViewportDescriptorDeclaration>