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/Forward.h

@@ -13,10 +13,13 @@ class ASTNode;
 class BetweenExpression;
 class BinaryOperatorExpression;
 class BlobLiteral;
+class BTree;
+class BTreeIterator;
 class CaseExpression;
 class CastExpression;
 class ChainedExpression;
 class CollateExpression;
+class ColumnDef;
 class ColumnDefinition;
 class ColumnNameExpression;
 class CommonTableExpression;
@@ -32,13 +35,19 @@ class ErrorStatement;
 class ExistsExpression;
 class Expression;
 class GroupByClause;
+class Heap;
 class InChainedExpression;
+class Index;
+class IndexNode;
+class IndexDef;
 class InSelectionExpression;
 class Insert;
 class InTableExpression;
 class InvertibleNestedDoubleExpression;
 class InvertibleNestedExpression;
 class IsExpression;
+class Key;
+class KeyPartDef;
 class Lexer;
 class LimitClause;
 class MatchExpression;
@@ -54,12 +63,15 @@ class RenameColumn;
 class RenameTable;
 class ResultColumn;
 class ReturningClause;
+class Row;
 class Select;
 class SignedNumber;
 class Statement;
 class StringLiteral;
+class TableDef;
 class TableOrSubquery;
 class Token;
+class TreeNode;
 class Tuple;
 class TypeName;
 class UnaryOperatorExpression;