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/layout/context.rs

@@ -8,7 +8,7 @@ use fnv::FnvHasher;
 use gfx::display_list::{WebRenderImageInfo, OpaqueNode};
 use gfx::font_cache_thread::FontCacheThread;
 use gfx::font_context::FontContext;
-use heapsize::HeapSizeOf;
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use msg::constellation_msg::PipelineId;
 use net_traits::image_cache::{CanRequestImages, ImageCache, ImageState};
 use net_traits::image_cache::{ImageOrMetadataAvailable, UsePlaceholder};
@@ -42,10 +42,10 @@ pub fn with_thread_local_font_context<F, R>(layout_context: &LayoutContext, f: F
     })
 }
 
-pub fn heap_size_of_persistent_local_context() -> usize {
+pub fn malloc_size_of_persistent_local_context(ops: &mut MallocSizeOfOps) -> usize {
     FONT_CONTEXT_KEY.with(|r| {
         if let Some(ref context) = *r.borrow() {
-            context.heap_size_of_children()
+            context.size_of(ops)
         } else {
             0
         }