Use Token::EOF instead of Option<Token> (#195)

This simplifies codes slightly, removing the need deal with the EOF case explicitly.

The clone kludge in `_ => self.expected("date/time field",
Token::Word(w.clone())))` will become unnecessary once we stop using
a separate match for the keywords, as suggested in
https://github.com/andygrove/sqlparser-rs/pull/193#issuecomment-641607194

Author: nickolay