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/properties/helpers.mako.rs

@@ -117,9 +117,7 @@
             use values::computed::ComputedVecIter;
 
             /// The computed value, effectively a list of single values.
-            #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-            #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-            #[derive(Clone, Debug, PartialEq)]
+            #[derive(Clone, Debug, MallocSizeOf, PartialEq)]
             % if need_animatable or animation_value_type == "ComputedValue":
             #[derive(Animate, ComputeSquaredDistance)]
             % endif
@@ -178,9 +176,7 @@
         }
 
         /// The specified value of ${name}.
-        #[derive(Clone, Debug, PartialEq)]
-        #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-        #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[derive(Clone, Debug, MallocSizeOf, PartialEq)]
         pub struct SpecifiedValue(pub Vec<single_value::SpecifiedValue>);
 
         impl ToCss for SpecifiedValue {
@@ -950,9 +946,7 @@
             pub type T = ::values::computed::${length_type};
         }
 
-        #[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
-        #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-        #[derive(Clone, Debug, PartialEq, ToCss)]
+        #[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss)]
         pub struct SpecifiedValue(pub ${length_type});
 
         % if length_type == "MozLength":