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>
Distinguishes parts that come from the parser and lexer. It used to be that all
the files were called "syntax*", but lexing and parsing are distinct phases that
are easier to keep track of when distinguished. syntax.yy.cpp being the source
file generated by flex, containing the lexer was particularly confusing, because
the yy tends to indicate it is a yacc/Bison product, and the ".tab." substring,
indicating "tables" is not really useful to the developer.
These names also match up with what Bison's C++ example uses, which will make
the transition easier.
Dave Abrahams
