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/script_layout_interface/reporter.rs

@@ -11,13 +11,13 @@ use servo_url::ServoUrl;
 use std::sync::{Mutex, Arc};
 use style::error_reporting::{ParseErrorReporter, ContextualParseError};
 
-#[derive(Clone, HeapSizeOf)]
+#[derive(Clone, MallocSizeOf)]
 pub struct CSSErrorReporter {
     pub pipelineid: PipelineId,
     // Arc+Mutex combo is necessary to make this struct Sync,
     // which is necessary to fulfill the bounds required by the
     // uses of the ParseErrorReporter trait.
-    #[ignore_heap_size_of = "Arc is defined in libstd"]
+    #[ignore_malloc_size_of = "Arc is defined in libstd"]
     pub script_chan: Arc<Mutex<IpcSender<ConstellationControlMsg>>>,
 }