Greatly improved expressions (#18)
- Expressions can now contain artithmetic for binary operations (+, -, *
and /).
- Expressions now follow correct operator pecedence.
- Added SQLSTATE 22012 "division by zero".
- Adding grouping expression (parenthesis).
- Added concatenation `||`.
- Added logical operators for `AND`, `OR` and `NOT`.
- Added unary operators `+` and `-`.
- Changes not equal operator `!=` to `<>` to correct follow the SQL
standard.
- Renamed the lexer tokens to be same as the naming conventions in the
SQL standard.
