[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/syntax_helpers.h

@@ -15,12 +15,6 @@
 
 namespace Carbon {
 
-// Initialized by main(), and used when printing errors.
-extern const char* input_filename;
-
-// Prints errors.
-void PrintSyntaxError(char* error, int line_num);
-
 // Runs the top-level declaration list.
 void ExecProgram(std::list<Declaration>* fs);