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/Index.h

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2021, Jan de Visser <jan@de-visser.net>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibCore/Object.h>
+#include <LibSQL/Forward.h>
+#include <LibSQL/Meta.h>
+
+namespace SQL {
+
+class IndexNode {
+public:
+    virtual ~IndexNode() = default;
+    [[nodiscard]] u32 pointer() const { return m_pointer; }
+    virtual void serialize(ByteBuffer&) const = 0;
+    virtual IndexNode* as_index_node() = 0;
+
+protected:
+    explicit IndexNode(u32 pointer)
+        : m_pointer(pointer)
+    {
+    }
+
+    void set_pointer(u32 pointer) { m_pointer = pointer; }
+
+private:
+    u32 m_pointer;
+};
+
+class Index : public Core::Object {
+    C_OBJECT_ABSTRACT(Index);
+
+public:
+    ~Index() override = default;
+
+    TupleDescriptor descriptor() const { return m_descriptor; }
+    [[nodiscard]] bool duplicates_allowed() const { return !m_unique; }
+    [[nodiscard]] bool unique() const { return m_unique; }
+    [[nodiscard]] u32 pointer() const { return m_pointer; }
+
+protected:
+    Index(Heap& heap, TupleDescriptor const&, bool unique, u32 pointer);
+    Index(Heap& heap, TupleDescriptor const&, u32 pointer);
+
+    [[nodiscard]] Heap const& heap() const { return m_heap; }
+    [[nodiscard]] Heap& heap() { return m_heap; }
+    void set_pointer(u32 pointer) { m_pointer = pointer; }
+    u32 new_record_pointer() { return m_heap.new_record_pointer(); }
+    ByteBuffer read_block(u32);
+    void add_to_write_ahead_log(IndexNode*);
+
+private:
+    Heap& m_heap;
+    TupleDescriptor m_descriptor;
+    bool m_unique { false };
+    u32 m_pointer { 0 };
+};
+
+}