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/stylesheet_set.rs

@@ -12,7 +12,7 @@ use std::slice;
 use stylesheets::{Origin, OriginSet, OriginSetIterator, PerOrigin, StylesheetInDocument};
 
 /// Entry for a StylesheetSet.
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 struct StylesheetSetEntry<S>
 where
     S: StylesheetInDocument + PartialEq + 'static,
@@ -103,7 +103,7 @@ where
 
 /// The validity of the data in a given cascade origin.
 #[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub enum OriginValidity {
     /// The origin is clean, all the data already there is valid, though we may
     /// have new sheets at the end.
@@ -260,7 +260,7 @@ where
     }
 }
 
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 struct SheetCollection<S>
 where
     S: StylesheetInDocument + PartialEq + 'static,
@@ -366,7 +366,7 @@ where
 }
 
 /// The set of stylesheets effective for a given document.
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
 pub struct StylesheetSet<S>
 where
     S: StylesheetInDocument + PartialEq + 'static,