style: Use a RwLock'd HashMap instead of a lock-free linked list for rule node children.

I need to profile this a bit more, but talos was pretty happy about this, and it
solves the known performance issues here such as the test-case from bug 1483963
for example. This also gets rid of a bunch of unsafe code which is nice.

This still keeps the same GC scheme, removing the key from the hashmap when
needed. I kept those as release assertions, but should probably be turned into
debug-only assertions.

Differential Revision: https://phabricator.services.mozilla.com/D6801

components/servo_arc/lib.rs

@@ -1305,11 +1305,18 @@ impl<A, B> ArcUnion<A, B> {
     }
 
     /// Returns true if the two values are pointer-equal.
+    #[inline]
     pub fn ptr_eq(this: &Self, other: &Self) -> bool {
         this.p == other.p
     }
 
+    #[inline]
+    pub fn ptr(&self) -> ptr::NonNull<()> {
+        self.p
+    }
+
     /// Returns an enum representing a borrow of either A or B.
+    #[inline]
     pub fn borrow(&self) -> ArcUnionBorrow<A, B> {
         if self.is_first() {
             let ptr = self.p.as_ptr() as *const A;