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/stylesheets/document_rule.rs

@@ -7,13 +7,15 @@
 //! We implement the prefixed `@-moz-document`.
 
 use cssparser::{Parser, Token, SourceLocation, BasicParseError};
+#[cfg(feature = "gecko")]
+use malloc_size_of::MallocSizeOfOps;
 use media_queries::Device;
 use parser::{Parse, ParserContext};
 use servo_arc::Arc;
 use shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked, SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
 use std::fmt;
 use style_traits::{ToCss, ParseError, StyleParseError};
-use stylesheets::{CssRules, MallocSizeOfFn, MallocSizeOfWithGuard};
+use stylesheets::CssRules;
 use values::specified::url::SpecifiedUrl;
 
 #[derive(Debug)]
@@ -29,10 +31,10 @@ pub struct DocumentRule {
 
 impl DocumentRule {
     /// Measure heap usage.
-    pub fn malloc_size_of_children(&self, guard: &SharedRwLockReadGuard,
-                                   malloc_size_of: MallocSizeOfFn) -> usize {
+    #[cfg(feature = "gecko")]
+    pub fn size_of(&self, guard: &SharedRwLockReadGuard, ops: &mut MallocSizeOfOps) -> usize {
         // Measurement of other fields may be added later.
-        self.rules.read_with(guard).malloc_size_of_children(guard, malloc_size_of)
+        self.rules.read_with(guard).size_of(guard, ops)
     }
 }