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/dom/webgltexture.rs

@@ -36,7 +36,7 @@ pub struct WebGLTexture {
     target: Cell<Option<u32>>,
     is_deleted: Cell<bool>,
     /// Stores information about mipmap levels and cubemap faces.
-    #[ignore_heap_size_of = "Arrays are cumbersome"]
+    #[ignore_malloc_size_of = "Arrays are cumbersome"]
     image_info_array: DomRefCell<[ImageInfo; MAX_LEVEL_COUNT * MAX_FACE_COUNT]>,
     /// Face count can only be 1 or 6
     face_count: Cell<u8>,
@@ -44,7 +44,7 @@ pub struct WebGLTexture {
     // Store information for min and mag filters
     min_filter: Cell<Option<u32>>,
     mag_filter: Cell<Option<u32>>,
-    #[ignore_heap_size_of = "Defined in ipc-channel"]
+    #[ignore_malloc_size_of = "Defined in ipc-channel"]
     renderer: WebGLMsgSender,
     /// True if this texture is used for the DOMToTexture feature.
     attached_to_dom: Cell<bool>,
@@ -394,7 +394,7 @@ impl Drop for WebGLTexture {
     }
 }
 
-#[derive(Clone, Copy, Debug, HeapSizeOf, JSTraceable, PartialEq)]
+#[derive(Clone, Copy, Debug, JSTraceable, MallocSizeOf, PartialEq)]
 pub struct ImageInfo {
     width: u32,
     height: u32,