[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/syntax/lexer.lpp

@@ -6,7 +6,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 %{
 #include <cstdlib>
-#include "executable_semantics/syntax/parser.h"
+#include "executable_semantics/syntax/parse_and_lex_context.h"
 %}
 
 /* Turn off legacy bits we don't need */