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/textinput.rs

@@ -22,7 +22,7 @@ pub enum Selection {
     NotSelected
 }
 
-#[derive(Clone, Copy, HeapSizeOf, JSTraceable, PartialEq)]
+#[derive(Clone, Copy, JSTraceable, MallocSizeOf, PartialEq)]
 pub enum SelectionDirection {
     Forward,
     Backward,
@@ -49,7 +49,7 @@ impl From<SelectionDirection> for DOMString {
     }
 }
 
-#[derive(Clone, Copy, HeapSizeOf, JSTraceable, PartialEq)]
+#[derive(Clone, Copy, JSTraceable, MallocSizeOf, PartialEq)]
 pub struct TextPoint {
     /// 0-based line number
     pub line: usize,
@@ -58,7 +58,7 @@ pub struct TextPoint {
 }
 
 /// Encapsulated state for handling keyboard input in a single or multiline text input control.
-#[derive(HeapSizeOf, JSTraceable)]
+#[derive(JSTraceable, MallocSizeOf)]
 pub struct TextInput<T: ClipboardProvider> {
     /// Current text input content, split across lines without trailing '\n'
     lines: Vec<DOMString>,
@@ -68,7 +68,7 @@ pub struct TextInput<T: ClipboardProvider> {
     pub selection_begin: Option<TextPoint>,
     /// Is this a multiline input?
     multiline: bool,
-    #[ignore_heap_size_of = "Can't easily measure this generic type"]
+    #[ignore_malloc_size_of = "Can't easily measure this generic type"]
     clipboard_provider: T,
     /// The maximum number of UTF-16 code units this text input is allowed to hold.
     ///