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

dom/html/HTMLAllCollection.cpp

@@ -112,7 +112,7 @@ HTMLAllCollection::GetDocumentAllList(const nsAString& aID)
 {
   return mNamedMap.LookupForAdd(aID).OrInsert(
     [this, &aID] () {
-      nsCOMPtr<nsIAtom> id = NS_Atomize(aID);
+      RefPtr<nsIAtom> id = NS_Atomize(aID);
       return new nsContentList(mDocument, DocAllResultMatch, nullptr,
                                nullptr, true, id);
     });