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/rule_tree/mod.rs

@@ -9,8 +9,6 @@
 use applicable_declarations::ApplicableDeclarationList;
 #[cfg(feature = "gecko")]
 use gecko::selector_parser::PseudoElement;
-#[cfg(feature = "servo")]
-use heapsize::HeapSizeOf;
 #[cfg(feature = "gecko")]
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use properties::{Importance, LonghandIdSet, PropertyDeclarationBlock};
@@ -46,7 +44,7 @@ use thread_state;
 /// logs from http://logs.glob.uno/?c=mozilla%23servo&s=3+Apr+2017&e=3+Apr+2017#c644094
 /// to se a discussion about the different memory orderings used here.
 #[derive(Debug)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub struct RuleTree {
     root: StrongRuleNode,
 }
@@ -463,7 +461,7 @@ const RULE_TREE_GC_INTERVAL: usize = 300;
 /// [1]: https://drafts.csswg.org/css-cascade/#cascade-origin
 #[repr(u8)]
 #[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub enum CascadeLevel {
     /// Normal User-Agent rules.
     UANormal = 0,
@@ -827,10 +825,7 @@ pub struct StrongRuleNode {
 }
 
 #[cfg(feature = "servo")]
-impl HeapSizeOf for StrongRuleNode {
-    fn heap_size_of_children(&self) -> usize { 0 }
-}
-
+malloc_size_of_is_0!(StrongRuleNode);
 
 impl StrongRuleNode {
     fn new(n: Box<RuleNode>) -> Self {