missing returnType of FunctionType

Hello, I wonder why [FunctionType](https://github.com/antlr/symtab/blob/master/src/org/antlr/symtab/FunctionType.java) don't provide a method to return `returnType` of `FunctionType`.

Also, the `toString` method of `FunctionType` also confuses me, maybe it's better to take `argumentTypes` into consideration?
```java
@Override
public String toString() {
return "*"+ returnType;
}
```

Start with src/org/antlr/symtab/FunctionType.java and read how returnType and argumentTypes are stored and represented. Confirm the expected accessor and string format from the existing class, then verify that the completed behavior exposes returnType and includes argumentTypes in toString().

Written by the indexing model from the issue text.