AK: Support range-for iteration over a NonnullRefPtrVector<T>.

This means you can now do this:

void harmonize(NonnullRefPtrVector<Voice>& voices)
{
    for (auto& voice : voices) {
        voice.sing(); // Look, no "->"!
    }
}

Pretty dang cool :^)

AK/NonnullRefPtrVector.h

@@ -20,6 +20,15 @@ public:
     }
 
     using Base::size;
+
+    using Iterator = VectorIterator<NonnullRefPtrVector, T>;
+    Iterator begin() { return Iterator(*this, 0); }
+    Iterator end() { return Iterator(*this, size()); }
+
+    using ConstIterator = ConstVectorIterator<NonnullRefPtrVector, T>;
+    ConstIterator begin() const { return ConstIterator(*this, 0); }
+    ConstIterator end() const { return ConstIterator(*this, size()); }
+
     T& at(int index) { return *Base::at(index); }
     const T& at(int index) const { return *Base::at(index); }
     T& operator[](int index) { return at(index); }