Bug 1400459 (part 2) - Devirtualize nsIAtom. r=heycam.

This patch merges nsAtom into nsIAtom. For the moment, both names can be used
interchangeably due to a typedef. The patch also devirtualizes nsIAtom, by
making it not inherit from nsISupports, removing NS_DECL_NSIATOM, and dropping
the use of NS_IMETHOD_. It also removes nsIAtom's IIDs.

These changes trigger knock-on changes throughout the codebase, changing the
types of lots of things as follows.

- nsCOMPtr<nsIAtom> --> RefPtr<nsIAtom>

- nsCOMArray<nsIAtom> --> nsTArray<RefPtr<nsIAtom>>
  - Count() --> Length()
  - ObjectAt() --> ElementAt()
  - AppendObject() --> AppendElement()
  - RemoveObjectAt() --> RemoveElementAt()

- ns*Hashtable<nsISupportsHashKey, ...> -->
  ns*Hashtable<nsRefPtrHashKey<nsIAtom>, ...>

- nsInterfaceHashtable<T, nsIAtom> --> nsRefPtrHashtable<T, nsIAtom>
  - This requires adding a Get() method to nsRefPtrHashtable that it lacks but
    nsInterfaceHashtable has.

- nsCOMPtr<nsIMutableArray> --> nsTArray<RefPtr<nsIAtom>>
  - nsArrayBase::Create() --> nsTArray()
  - GetLength() --> Length()
  - do_QueryElementAt() --> operator[]

The patch also has some changes to Rust code that manipulates nsIAtom.

MozReview-Commit-ID: DykOl8aEnUJ

widget/windows/KeyboardLayout.cpp

@@ -1975,7 +1975,7 @@ NativeKey::MaybeInitPluginEventOfKeyEvent(WidgetKeyboardEvent& aKeyEvent,
 bool
 NativeKey::DispatchCommandEvent(uint32_t aEventCommand) const
 {
-  nsCOMPtr<nsIAtom> command;
+  RefPtr<nsIAtom> command;
   switch (aEventCommand) {
     case APPCOMMAND_BROWSER_BACKWARD:
       command = nsGkAtoms::Back;