Add a DoubleBuffer thingy to allow TTY read/write to be interleaved.

I feel like this concept might be useful in more places. It's very naive
right now and uses dynamically growing buffers. It should really use static
size buffers and never kmalloc().

AK/Vector.h

@@ -188,6 +188,14 @@ public:
         ++m_impl->m_size;
     }
 
+    void append(const T* values, size_t count)
+    {
+        ensureCapacity(size() + count);
+        for (size_t i = 0; i < count; ++i)
+            new (m_impl->slot(m_impl->m_size + i)) T(values[i]);
+        m_impl->m_size += count;
+    }
+
     void ensureCapacity(size_t neededCapacity)
     {
         if (capacity() >= neededCapacity)