The parser state does not just necessarily include the position of the
buffer, but make it possible to maintain additional state.
Furthermore, add an explicit ability to push back a token.
All of this might make it easier at some point in the future to keep
track of horizontal position, although that will require lots of
changes to the preprocessor.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
7cc9fc4773