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/geometry/lib.rs

@@ -4,7 +4,8 @@
 
 extern crate app_units;
 extern crate euclid;
-#[macro_use] extern crate heapsize;
+extern crate malloc_size_of;
+#[macro_use] extern crate malloc_size_of_derive;
 
 use app_units::{Au, MAX_AU, MIN_AU};
 use euclid::{Point2D, Rect, Size2D};
@@ -24,11 +25,9 @@ use euclid::{Point2D, Rect, Size2D};
 ///
 /// The ratio between DeviceIndependentPixel and DevicePixel for a given display be found by calling
 /// `servo::windowing::WindowMethods::hidpi_factor`.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, MallocSizeOf)]
 pub enum DeviceIndependentPixel {}
 
-known_heap_size!(0, DeviceIndependentPixel);
-
 // An Au is an "App Unit" and represents 1/60th of a CSS pixel.  It was
 // originally proposed in 2002 as a standard unit of measure in Gecko.
 // See https://bugzilla.mozilla.org/show_bug.cgi?id=177805 for more info.