AK: Make it possible to use HashMap<K, NonnullOwnPtr>::get()

Add the concept of a PeekType to Traits<T>. This is the type we'll
return (wrapped in an Optional) from HashMap::get().

The PeekType for OwnPtr<T> and NonnullOwnPtr<T> is const T*,
which means that HashMap::get() will return an Optional<const T*> for
maps-of-those.

AK/RefPtr.h

@@ -183,6 +183,12 @@ public:
         return exchange(m_ptr, nullptr);
     }
 
+    NonnullRefPtr<T> release_nonnull()
+    {
+        ASSERT(m_ptr);
+        return NonnullRefPtr<T>(NonnullRefPtr<T>::Adopt, *leak_ref());
+    }
+
     T* ptr() { return m_ptr; }
     const T* ptr() const { return m_ptr; }