IN PROGRESS - issue DRUID-81: SQLExpr的派生类实现Equals和HashCode方法

http://code.alibabatech.com/jira/browse/DRUID-81

src/main/java/com/alibaba/druid/sql/ast/SQLExprImpl.java

@@ -22,5 +22,8 @@ public abstract class SQLExprImpl extends SQLObjectImpl implements SQLExpr {
     public SQLExprImpl(){
 
     }
+    
+    public abstract boolean equals(Object o);
 
+    public abstract int hashCode();
 }