[executable semantics] Add Syntax driver (#362)

Slowly bringing this into line with Bison's C++ example parser
so we can use strong semantic values for symbols rather than
leaking pointers.  First step is to thread a `ParseAndLexContext` 
object through the whole syntactic analysis state, like the 
example has.  In the example, it's called `driver`.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/ast/abstract_syntax_tree.h

@@ -0,0 +1,16 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_AST_ABSTRACT_SYNTAX_TREE_H_
+#define EXECUTABLE_SEMANTICS_AST_ABSTRACT_SYNTAX_TREE_H_
+
+#include <variant>
+
+#include "executable_semantics/ast/declaration.h"
+
+namespace Carbon {
+using AST = std::list<Carbon::Declaration>*;
+}
+
+#endif  // EXECUTABLE_SEMANTICS_AST_ABSTRACT_SYNTAX_TREE_H_