LibSQL: BTree index, Heap, and Meta objects for SQL Storage layer

Unfortunately this patch is quite large.

The main functionality included are a BTree index implementation and
the Heap class which manages persistent storage.

Also included are a Key subclass of the Tuple class, which is a
specialization for index key tuples. This "dragged in" the Meta layer,
which has classes defining SQL objects like tables and indexes.

Userland/Libraries/LibSQL/Key.cpp

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2021, Jan de Visser <jan@de-visser.net>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibSQL/Key.h>
+#include <LibSQL/Meta.h>
+
+namespace SQL {
+
+Key::Key()
+    : Tuple()
+{
+}
+
+Key::Key(TupleDescriptor const& descriptor)
+    : Tuple(descriptor)
+{
+}
+
+Key::Key(RefPtr<IndexDef> index)
+    : Tuple(index->to_tuple_descriptor())
+    , m_index(index)
+{
+}
+
+Key::Key(TupleDescriptor const& descriptor, ByteBuffer& buffer, size_t& offset)
+    : Tuple(descriptor, buffer, offset)
+{
+}
+
+Key::Key(RefPtr<IndexDef> index, ByteBuffer& buffer, size_t& offset)
+    : Key(index->to_tuple_descriptor())
+{
+    deserialize(buffer, offset);
+}
+
+}