Overhaul MallocSizeOf and related things.

This patch makes the MallocSizeOf stuff in Stylo work more like the HeapSizeOf
stuff already in Servo, except better. In particular, it adds deriving support
for MallocSizeOf, which will make it easier to improve coverage.

The patch does the following.

- Combines servo/components/style/stylesheets/memory.rs and the heapsize crate
  into a new crate, malloc_size_of.

- Forks the heapsize_derive crate, calling it malloc_size_of, so that
  MallocSizeOf can be derived.

- Both the new crates have MIT/Apache licenses, like heapsize, in case they are
  incorporated into heapsize in the future.

- Renames the methods within MallocSizeOf and the related traits so they are
  more concise.

- Removes MallocSizeOfWithGuard.

- Adds `derive(MallocSizeOf)` to a lot of types, in some cases replacing an
  equivalent or almost-equivalent hand-written implementation.

- Adds stuff so that Rc/Arc can be handled properly.

components/style/rule_tree/mod.rs

@@ -9,6 +9,8 @@
 use applicable_declarations::ApplicableDeclarationList;
 #[cfg(feature = "servo")]
 use heapsize::HeapSizeOf;
+#[cfg(feature = "gecko")]
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use properties::{Importance, LonghandIdSet, PropertyDeclarationBlock};
 use servo_arc::{Arc, ArcBorrow, NonZeroPtrMut};
 use shared_lock::{Locked, StylesheetGuards, SharedRwLockReadGuard};
@@ -17,7 +19,7 @@ use std::io::{self, Write};
 use std::mem;
 use std::ptr;
 use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
-use stylesheets::{MallocSizeOf, MallocSizeOfFn, StyleRule};
+use stylesheets::StyleRule;
 use thread_state;
 
 /// The rule tree, the structure servo uses to preserve the results of selector
@@ -62,9 +64,12 @@ impl Drop for RuleTree {
     }
 }
 
+#[cfg(feature = "gecko")]
 impl MallocSizeOf for RuleTree {
-    fn malloc_size_of_children(&self, malloc_size_of: MallocSizeOfFn) -> usize {
-        self.root.get().malloc_size_of_including_self(malloc_size_of)
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        let mut n = ops.malloc_size_of(self.root.ptr());
+        n += self.root.get().size_of(ops);
+        n
     }
 }
 
@@ -794,11 +799,15 @@ impl RuleNode {
             }
         }
     }
+}
 
-    fn malloc_size_of_including_self(&self, malloc_size_of: MallocSizeOfFn) -> usize {
-        let mut n = unsafe { (malloc_size_of.0)(self as *const _ as *const _) };
+#[cfg(feature = "gecko")]
+impl MallocSizeOf for RuleNode {
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        let mut n = 0;
         for child in self.iter_children() {
-            n += unsafe { (*child.ptr()).malloc_size_of_including_self(malloc_size_of) };
+            n += ops.malloc_size_of(child.ptr());
+            n += unsafe { (*child.ptr()).size_of(ops) };
         }
         n
     }