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/devtools_traits/lib.rs

@@ -14,10 +14,10 @@
 
 #[macro_use]
 extern crate bitflags;
-extern crate heapsize;
-#[macro_use] extern crate heapsize_derive;
 extern crate hyper;
 extern crate ipc_channel;
+extern crate malloc_size_of;
+#[macro_use] extern crate malloc_size_of_derive;
 extern crate msg;
 #[macro_use] extern crate serde;
 extern crate servo_url;
@@ -40,7 +40,7 @@ pub struct DevtoolsPageInfo {
     pub url: ServoUrl,
 }
 
-#[derive(Clone, Debug, Deserialize, HeapSizeOf, Serialize)]
+#[derive(Clone, Debug, Deserialize, MallocSizeOf, Serialize)]
 pub struct CSSError {
     pub filename: String,
     pub line: u32,
@@ -144,7 +144,7 @@ pub struct TimelineMarker {
     pub end_stack: Option<Vec<()>>,
 }
 
-#[derive(Clone, Debug, Deserialize, Eq, Hash, HeapSizeOf, PartialEq, Serialize)]
+#[derive(Clone, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
 pub enum TimelineMarkerType {
     Reflow,
     DOMEvent,
@@ -355,5 +355,5 @@ impl PreciseTime {
     }
 }
 
-#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, HeapSizeOf, PartialEq, Serialize)]
+#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
 pub struct WorkerId(pub u32);