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/length.rs

@@ -52,9 +52,7 @@ pub fn au_to_int_px(au: f32) -> i32 {
     (au / AU_PER_PX).round() as i32
 }
 
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq)]
 /// A font relative length.
 pub enum FontRelativeLength {
     /// A "em" value: https://drafts.csswg.org/css-values/#em
@@ -208,9 +206,7 @@ impl FontRelativeLength {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq)]
 /// A viewport-relative length.
 ///
 /// <https://drafts.csswg.org/css-values/#viewport-relative-lengths>
@@ -259,9 +255,7 @@ impl ViewportPercentageLength {
 }
 
 /// HTML5 "character width", as defined in HTML5 § 14.5.4.
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq)]
 pub struct CharacterWidth(pub i32);
 
 impl CharacterWidth {
@@ -279,9 +273,7 @@ impl CharacterWidth {
 }
 
 /// Represents an absolute length with its unit
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq)]
 pub enum AbsoluteLength {
     /// An absolute length in pixels (px)
     Px(CSSFloat),
@@ -441,9 +433,7 @@ impl Mul<CSSFloat> for PhysicalLength {
 /// A `<length>` without taking `calc` expressions into account
 ///
 /// <https://drafts.csswg.org/css-values/#lengths>
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq)]
 pub enum NoCalcLength {
     /// An absolute length
     ///
@@ -582,9 +572,7 @@ impl NoCalcLength {
 /// This is commonly used for the `<length>` values.
 ///
 /// <https://drafts.csswg.org/css-values/#lengths>
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToCss)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
 pub enum Length {
     /// The internal length type that cannot parse `calc`
     NoCalc(NoCalcLength),
@@ -798,9 +786,7 @@ pub type NonNegativeLengthOrNumber = Either<NonNegativeLength, NonNegativeNumber
 
 /// A length or a percentage value.
 #[allow(missing_docs)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToCss)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
 pub enum LengthOrPercentage {
     Length(NoCalcLength),
     Percentage(computed::Percentage),
@@ -970,9 +956,7 @@ impl LengthOrPercentage {
 
 /// Either a `<length>`, a `<percentage>`, or the `auto` keyword.
 #[allow(missing_docs)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToCss)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
 pub enum LengthOrPercentageOrAuto {
     Length(NoCalcLength),
     Percentage(computed::Percentage),
@@ -1088,9 +1072,7 @@ impl LengthOrPercentageOrAuto {
 }
 
 /// Either a `<length>`, a `<percentage>`, or the `none` keyword.
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToCss)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
 #[allow(missing_docs)]
 pub enum LengthOrPercentageOrNone {
     Length(NoCalcLength),
@@ -1238,9 +1220,7 @@ impl LengthOrNumber {
 /// Unlike `max-width` or `max-height` properties, a MozLength can be
 /// `auto`, and cannot be `none`.
 #[allow(missing_docs)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToCss)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
 pub enum MozLength {
     LengthOrPercentageOrAuto(LengthOrPercentageOrAuto),
     ExtremumLength(ExtremumLength),
@@ -1265,9 +1245,7 @@ impl MozLength {
 
 /// A value suitable for a `max-width` or `max-height` property.
 #[allow(missing_docs)]
-#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToCss)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
 pub enum MaxLength {
     LengthOrPercentageOrNone(LengthOrPercentageOrNone),
     ExtremumLength(ExtremumLength),