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/specified/mod.rs

@@ -176,9 +176,7 @@ impl BorderStyle {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, PartialOrd)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, PartialOrd)]
 #[allow(missing_docs)]
 pub struct Number {
     /// The numeric value itself.
@@ -294,9 +292,7 @@ impl Parse for GreaterThanOrEqualToOneNumber {
 ///
 /// FIXME(emilio): Should probably use Either.
 #[allow(missing_docs)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, ToCss)]
 pub enum NumberOrPercentage {
     Percentage(Percentage),
     Number(Number),
@@ -330,9 +326,7 @@ impl Parse for NumberOrPercentage {
 }
 
 #[allow(missing_docs)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, PartialEq, PartialOrd, ToCss)]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, PartialOrd, ToCss)]
 pub struct Opacity(Number);
 
 
@@ -366,9 +360,7 @@ impl ToComputedValue for Opacity {
 /// An specified `<integer>`, optionally coming from a `calc()` expression.
 ///
 /// <https://drafts.csswg.org/css-values/#integers>
-#[derive(Clone, Copy, Debug, PartialEq, PartialOrd)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, PartialOrd)]
 pub struct Integer {
     value: CSSInteger,
     was_calc: bool,
@@ -521,9 +513,7 @@ pub type LengthOrPercentageOrNumber = Either<Number, LengthOrPercentage>;
 /// NonNegativeLengthOrPercentage | NonNegativeNumber
 pub type NonNegativeLengthOrPercentageOrNumber = Either<NonNegativeNumber, NonNegativeLengthOrPercentage>;
 
-#[derive(Clone, Debug, PartialEq)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
 /// rect(<top>, <left>, <bottom>, <right>) used by clip and image-region
 pub struct ClipRect {
     /// <top> (<length> | <auto>)
@@ -693,9 +683,7 @@ pub type NamespaceId = ();
 /// An attr(...) rule
 ///
 /// `[namespace? `|`]? ident`
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, Eq, PartialEq, ToComputedValue)]
+#[derive(Clone, Debug, Eq, MallocSizeOf, PartialEq, ToComputedValue)]
 pub struct Attr {
     /// Optional namespace
     pub namespace: Option<(Namespace, NamespaceId)>,