Support negative integers in expressions

[theodesp]

Currently, any operation that involves negative numbers fail. For example:

mysql> SELECT CEIL(-1.23);
ERROR 1105 (HY000): unknown error: unsupported syntax: &sqlparser.UnaryExpr{Operator:"-", Expr:(*sqlparser.SQLVal)(0xc4203cfc60)}
mysql> SELECT FLOOR(-1.23);
ERROR 1105 (HY000): unknown error: unsupported syntax: &sqlparser.UnaryExpr{Operator:"-", Expr:(*sqlparser.SQLVal)(0xc4203cfde0)}